(:zap: may break some mrbgems)
- change format of MRUBY_VERSION
- mruby.h includes mruby/version.h
- remove support for very old cygwin releases
- add RUBY_ENGINE (C API)
- ⚡ add mrb_module_get / mrb_module_get_under
- change mrb_define_class (C API)
- ⚡ remove mrb_name_class
- add MRUBY_RELEASE_NO (C API)
- add MRUBY_RUBY_VERSION and other constants (C API)
- add mrb_str_new_lit (C API)
- add RUBY_VERSION to notify syntax compatible version
- add Float#nan?
- ⚡ clean up external symbols
- ⚡ mrb_str_format() has moved to mruby-sprintf