inline structures data type for mruby (MRB_TT_INLINE) (fix #3237)

Inline structures have no instance variables, no finalizer, and offer as much space as possible in RBASIC object. This means 24 bytes on 64-bit platforms and 12 bytes on 32-bit platforms.
mruby-inline-struct gem is only provided for testing.

include/mruby/inline.h

@@ -0,0 +1,47 @@
+/*
+** mruby/inline.h - Inline structures
+**
+** See Copyright Notice in mruby.h
+*/
+
+#ifndef MRUBY_INLINE_H
+#define MRUBY_INLINE_H
+
+#include "common.h"
+#include <string.h>
+
+/**
+ * Inline structures that fit in RVALUE
+ *
+ * They cannot have finalizer, and cannot have instance variables.
+ */
+MRB_BEGIN_DECL
+
+#define INLINE_DATA_SIZE (sizeof(void*) * 3)
+
+struct RInline {
+  MRB_OBJECT_HEADER;
+  char inline_data[INLINE_DATA_SIZE];
+};
+
+#define RINLINE(obj)         ((struct RInline*)(mrb_ptr(obj)))
+#define INLINE_PTR(obj)      (RINLINE(obj)->inline_data)
+
+MRB_INLINE mrb_int mrb_inline_size()
+{
+  return INLINE_DATA_SIZE;
+}
+
+MRB_INLINE void* mrb_inline_ptr(mrb_value object)
+{
+  return INLINE_PTR(object);
+}
+
+MRB_INLINE void mrb_inline_copy(mrb_value dest, mrb_value src)
+{
+  memcpy(INLINE_PTR(dest), INLINE_PTR(src), INLINE_DATA_SIZE);
+}
+
+MRB_END_DECL
+
+#endif /* MRUBY_INLINE_H */

include/mruby/value.h

@@ -116,7 +116,8 @@ enum mrb_vtype {
   MRB_TT_ENV,         /*  20 */
   MRB_TT_DATA,        /*  21 */
   MRB_TT_FIBER,       /*  22 */
-  MRB_TT_MAXDEFINE    /*  23 */
+  MRB_TT_INLINE,      /*  23 */
+  MRB_TT_MAXDEFINE    /*  24 */
 };
 
 #include <mruby/object.h>