Experiment: ResumableParser

Fix: #983

Numerous known issues TODO:

  - `object_start_cursor` recorded in frame becomes invalid if the buffer string
    is reallocated or spilled.
  - The buffer need to be shrunk sometimes.
  - Lot more testing needed.
  - Unclear what to do with top level numbers (and perhaps true/false/null)
  - API is all but final
    - I'd like to be able to "pop" the value, so we don't uselessly keep a
      reference on it.
    - Then methods need to be documented.
  - It would worth trying to make `json_parse_any` exception free.
    - Right now EOF errors have been eliminated in favor of returning
      `false`.
    - We could try to do the same with syntax errors.
    - But then we need to `rb_protect` when calling back into Ruby
      or other unsafe APIs, so perhaps it's best to just accept it.

Author: byroot

ext/json/ext/json.h

@@ -11,6 +11,9 @@
 
 #if defined(RUBY_DEBUG) && RUBY_DEBUG
 # define JSON_ASSERT RUBY_ASSERT
+# ifndef JSON_DEBUG
+#  define JSON_DEBUG 1
+# endif
 #else
 # ifdef JSON_DEBUG
 #  include <assert.h>
@@ -20,8 +23,18 @@
 # endif
 #endif
 
+#ifdef JSON_DEBUG
+# define JSON_UNREACHABLE_RETURN(val) rb_bug("Unreachable")
+#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)