add mrb_intern_lit for creating symbol from string literal
Merge pull request #1600 from take-cheeze/mrb_intern_lit

Proposal of mrb_intern_lit macro.
Change scapa and rcapa to size_t, use scapa.
Merge pull request #1601 from carsonmcdonald/srcaptosizet

Change scapa and rcapa to size_t, use scapa.
should not access scope after releasing memory pool
avoid top-level pool release at codegen_error() and fixed some memory leaks