json: remove use of the restrict qualifier

This mostly just complicates type manipulations. We can add it back in
if performance actually warrants it, or if it becomes an official
qualifier.

json/flat.cpp.rl

@@ -180,9 +180,7 @@ json::flat::parse (util::view<KLASS>);
 MAP0(INSTANTIATE,
     std::string::iterator,
     std::string::const_iterator,
-    const char* restrict,
     const char*,
-    char *restrict,
     char *
 )
 

json/tree.cpp

@@ -230,9 +230,7 @@ json::tree::parse (const util::view<KLASS>&);
 MAP0(INSTANTIATE,
     std::string::iterator,
     std::string::const_iterator,
-    const char* restrict,
     const char*,
-    char* restrict,
     char *
 );