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/tests/function/builtin/testscript b/tests/function/builtin/testscript
index 88f802a..04e8bd8 100644
--- a/tests/function/builtin/testscript
+++ b/tests/function/builtin/testscript
@@ -53,6 +53,9 @@
   $* <'print $empty(abc)'          >'false'  : name
   $* <'print $empty(abc cxx{foo})' >'false'  : names
   $* <'print $empty([bool] false)' >'false'  : bool
+  $* <'print $empty([json] null)'  >'true'   : json-null
+  $* <'print $empty([json] "[]")'  >'true'   : json-array
+  $* <'print $empty([json] "{}")'  >'true'   : json-object
 }
 
 : first-second