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, 3 insertions, 0 deletions

diff --git a/libbuild2/function.cxx b/libbuild2/function.cxx
index f979d92..3110547 100644
--- a/libbuild2/function.cxx
+++ b/libbuild2/function.cxx
@@ -355,6 +355,7 @@ namespace build2
   void builtin_functions (function_map&);        // functions-builtin.cxx
   void filesystem_functions (function_map&);     // functions-filesystem.cxx
   void integer_functions (function_map&);        // functions-integer.cxx
+  void json_functions (function_map&);           // functions-json.cxx
   void name_functions (function_map&);           // functions-name.cxx
   void path_functions (function_map&);           // functions-path.cxx
   void process_functions (function_map&);        // functions-process.cxx
@@ -365,6 +366,7 @@ namespace build2
   void target_triplet_functions (function_map&); // functions-target-triplet.cxx
   void project_name_functions (function_map&);   // functions-target-triplet.cxx
 
+
   void
   insert_builtin_functions (function_map& m)
   {
@@ -372,6 +374,7 @@ namespace build2
     builtin_functions (m);
     filesystem_functions (m);
     integer_functions (m);
+    json_functions (m);
     name_functions (m);
     path_functions (m);
     process_functions (m);