[ruby/json] Implement JSON::ResumableParser

Only available on MRI for now.

ruby/json@2b908f6f26

Author: byroot

ext/json/json.h

@@ -28,8 +28,13 @@
 #else
 # define JSON_UNREACHABLE_RETURN UNREACHABLE_RETURN
 #endif
+
 /* shims */
 
+#ifndef UNDEF_P
+#define UNDEF_P(val) (val == Qundef)
+#endif
+
 #if SIZEOF_UINT64_T == SIZEOF_LONG_LONG
 # define INT64T2NUM(x) LL2NUM(x)
 # define UINT64T2NUM(x) ULL2NUM(x)