fix: broken variable reference, use 'next' statement

fix mruby exit status with mrb file.

 Because the tree is gone pop() even if it is empty.

fixes *.mrb dump/load format with escaped character.

fix cannot get/set const variable form singleton class

Use NULL instead of 0.

 - hex-style string support
 - mrb format changes like:
  - "\n" (before: \n -> after: \n)
  - '\n' (before: \n -> after: \\n)

IO.printf returns nil on CRuby

Use substitution instead of memset in structure initialization.

define convert method mrb_int/mrb_float with C string

change the error message (NoMethodError, NameError)

Use substitution instead of memset in structure initializations.

Change C API for raise.

mrb_raise(): API modified. It cannot treat variable arguments.

Remove redundant sizeof(char).

Fix prototype/declaration mismatch in uint8_dump().

Avoid memcpy() on copying structures.

Avoid memcpy() on copying structures.