Add experimental support for JSON value types

New types:

  json
  json_array
  json_object

New functions:

  $json.value_type(<json>)
  $json.value_size(<json>)
  $json.member_{name,value}(<json-member>)
  $json.object_names(<json-object>)
  $json.array_size(<json-array>)
  $json.array_find(<json-array>, <json>)
  $json.array_find_index(<json-array>, <json>)
  $json.load(<path>)
  $json.parse(<text>)
  $json.serialize(<json>[, <indentation>])

For example, to load a JSON value from a file:

j = $json.load($src_base/board.json)

Or to construct it in a buildfile:

j = [json] one@1 two@([json] 2 3 4) three@([json] x@1 y@-1)

This can also be done incrementally with append/prepend:

j = [json_object]
j += one@1
j += two@([json] 2 3 4)
j += three@([json] x@1 y@-1)

Instead of using this JSON-like syntax, one can also specify valid JSON
input text:

j = [json] '{"one":1, "two":[2, 3, 4], "three":{"x":1, "y":-1}'

Besides the above set of functions, other handy ways to access components
in a JSON value are iteration and subscript. For example:

for m: $j
  print $member_name($m) $member_value($m)

print ($j[three])

A subscript can be nested:

print ($j[two][1])
print ($j[three][x])

While a JSON value can be printed directly like any other value, the
representation will not be pretty-printed. As a result, for complex
JSON values, printing a serialized representation might be a more
readable option:

info $serialize($j)

1 files changed, 100 insertions, 24 deletions

diff --git a/libbuild2/parser.cxx b/libbuild2/parser.cxx
index 043cd10..1ac159b 100644
--- a/libbuild2/parser.cxx
+++ b/libbuild2/parser.cxx
@@ -5761,30 +5761,106 @@ namespace build2
   const value_type* parser::
   find_value_type (const scope*, const string& n)
   {
-    auto ptr = [] (const value_type& vt) {return &vt;};
-
-    return
-      n == "bool"           ? ptr (value_traits<bool>::value_type)           :
-      n == "int64"          ? ptr (value_traits<int64_t>::value_type)        :
-      n == "uint64"         ? ptr (value_traits<uint64_t>::value_type)       :
-      n == "string"         ? ptr (value_traits<string>::value_type)         :
-      n == "path"           ? ptr (value_traits<path>::value_type)           :
-      n == "dir_path"       ? ptr (value_traits<dir_path>::value_type)       :
-      n == "abs_dir_path"   ? ptr (value_traits<abs_dir_path>::value_type)   :
-      n == "name"           ? ptr (value_traits<name>::value_type)           :
-      n == "name_pair"      ? ptr (value_traits<name_pair>::value_type)      :
-      n == "target_triplet" ? ptr (value_traits<target_triplet>::value_type) :
-      n == "project_name"   ? ptr (value_traits<project_name>::value_type)   :
-
-      n == "int64s"         ? ptr (value_traits<int64s>::value_type)         :
-      n == "uint64s"        ? ptr (value_traits<uint64s>::value_type)        :
-      n == "strings"        ? ptr (value_traits<strings>::value_type)        :
-      n == "paths"          ? ptr (value_traits<paths>::value_type)          :
-      n == "dir_paths"      ? ptr (value_traits<dir_paths>::value_type)      :
-      n == "names"          ? ptr (value_traits<vector<name>>::value_type)   :
-      n == "cmdline"        ? ptr (value_traits<cmdline>::value_type)        :
-
-      nullptr;
+    switch (n[0])
+    {
+    case 'a':
+      {
+        if (n == "abs_dir_path") return &value_traits<abs_dir_path>::value_type;
+        break;
+      }
+    case 'b':
+      {
+        if (n == "bool") return &value_traits<bool>::value_type;
+        break;
+      }
+    case 'c':
+      {
+        if (n == "cmdline") return &value_traits<cmdline>::value_type;
+        break;
+      }
+    case 'd':
+      {
+        if (n.compare (0, 8, "dir_path") == 0)
+        {
+          if (n[8] == '\0') return &value_traits<dir_path>::value_type;
+          if (n[8] == 's' &&
+              n[9] == '\0') return &value_traits<dir_paths>::value_type;
+        }
+        break;
+      }
+    case 'i':
+      {
+        if (n.compare (0, 5, "int64") == 0)
+        {
+          if (n[5] == '\0') return &value_traits<int64_t>::value_type;
+          if (n[5] == 's' &&
+              n[6] == '\0') return &value_traits<int64s>::value_type;
+        }
+        break;
+      }
+    case 'j':
+      {
+        if (n.compare (0, 4, "json") == 0)
+        {
+          if (n[4] == '\0') return &value_traits<json_value>::value_type;
+          if (n == "json_array") return &value_traits<json_array>::value_type;
+          if (n == "json_object") return &value_traits<json_object>::value_type;
+        }
+        break;
+      }
+    case 'n':
+      {
+        if (n.compare (0, 4, "name") == 0)
+        {
+          if (n[4] == '\0') return &value_traits<name>::value_type;
+          if (n[4] == 's' &&
+              n[5] == '\0') return &value_traits<vector<name>>::value_type;
+          if (n == "name_pair") return &value_traits<name_pair>::value_type;
+        }
+        break;
+      }
+
+    case 'p':
+      {
+        if (n.compare (0, 4, "path") == 0)
+        {
+          if (n[4] == '\0') return &value_traits<path>::value_type;
+          if (n[4] == 's' &&
+              n[5] == '\0') return &value_traits<paths>::value_type;
+        }
+        else if (n == "project_name") return &value_traits<project_name>::value_type;
+        break;
+      }
+    case 's':
+      {
+        if (n.compare (0, 6, "string") == 0)
+        {
+          if (n[6] == '\0') return &value_traits<string>::value_type;
+          if (n[6] == 's' &&
+              n[7] == '\0') return &value_traits<strings>::value_type;
+        }
+        break;
+      }
+    case 't':
+      {
+        if (n == "target_triplet") return &value_traits<target_triplet>::value_type;
+        break;
+      }
+    case 'u':
+      {
+        if (n.compare (0, 6, "uint64") == 0)
+        {
+          if (n[6] == '\0') return &value_traits<uint64_t>::value_type;
+          if (n[6] == 's' &&
+              n[7] == '\0') return &value_traits<uint64s>::value_type;
+        }
+        break;
+      }
+    default:
+      break;
+    }
+
+    return nullptr;
   }
 
   void parser::