add new functions to check if symbols are interned without registration.
modify the type of return value (mrb_bool => mrb_value).
modify functions to suppress symbol registration into intern pool.
suppress intern inside of 'respond_to?'.
Merge branch 'suppress_intern' of https://github.com/crimsonwoods/mruby into crimsonwoods-suppress_intern
rename mrb_interned API functions
small cosmetic refactoring
Merge branch 'master' of github.com:mruby/mruby