Add json_map and json_set buildfile value types

These expose the std::map<json_value,json_value> and std::set<json_value>
types to buildfiles.

New functions:

  $size(<json-set>)
  $size(<json-map>)
  $keys(<json-map>)

Note that the $keys() function returns the list of map key as a json array.

For example:

m = [json_map] 2@([json] a@1 b@2) 1@([json] 1 2)
s = [json_set] ([json] x@1 y@2) ([json] a@1 b@2)

print ($m[2][b])              # 2
print ($s[([json] y@2 x@1)])  # true

1 files changed, 41 insertions, 1 deletions

diff --git a/libbuild2/variable.ixx b/libbuild2/variable.ixx
index a448cd8..ca84a33 100644
--- a/libbuild2/variable.ixx
+++ b/libbuild2/variable.ixx
@@ -362,13 +362,53 @@ namespace build2
 
   // This one will be SFINAE'd out unless T is a container.
   //
+  // If T is both (e.g., json_value), then make this version preferable.
+  //
   template <typename T>
   inline auto
-  convert (names&& ns) -> decltype (value_traits<T>::convert (move (ns)))
+  convert_impl (names&& ns, int)
+    -> decltype (value_traits<T>::convert (move (ns)))
   {
     return value_traits<T>::convert (move (ns));
   }
 
+  // This one will be SFINAE'd out unless T is a simple value.
+  //
+  // If T is both (e.g., json_value), then make this version less preferable.
+  //
+  template <typename T>
+  auto // NOTE: not inline!
+  convert_impl (names&& ns, ...) ->
+    decltype (value_traits<T>::convert (move (ns[0]), nullptr))
+  {
+    size_t n (ns.size ());
+
+    if (n == 0)
+    {
+      if (value_traits<T>::empty_value)
+        return T ();
+    }
+    else if (n == 1)
+    {
+      return convert<T> (move (ns[0]));
+    }
+    else if (n == 2 && ns[0].pair != '\0')
+    {
+      return convert<T> (move (ns[0]), move (ns[1]));
+    }
+
+    throw invalid_argument (
+      string ("invalid ") + value_traits<T>::type_name +
+      (n == 0 ? " value: empty" : " value: multiple names"));
+  }
+
+  template <typename T>
+  inline T
+  convert (names&& ns)
+  {
+    return convert_impl<T> (move (ns), 0);
+  }
+
   // bool value
   //
   inline void value_traits<bool>::