String nodes holds ruby string object on `VALUE nd_lit`.
This commit changes it to `struct rb_parser_string *string`
to reduce dependency on ruby object.
Sometimes these strings are concatenated with other string
therefore string concatenate functions are needed.
NODE_ARGS, NODE_ARYPTN, NODE_FNDPTN manage memory of their
structure by imemo tmpbuf Object.
However rb_ast_struct has reference to NODE. Then these
memory can be freed directly when rb_ast_struct is freed.
This commit reduces parser's dependency on CRuby functions.
Introduce Universal Parser mode for the parser.
This commit includes these changes:
* Introduce `UNIVERSAL_PARSER` macro. All of CRuby related functions
are passed via `struct rb_parser_config_struct` when this macro is enabled.
* Add CI task with 'cppflags=-DUNIVERSAL_PARSER' for ubuntu.
