Introduce mrb_check_convert_type_static

`mrb_check_convert_type` is a little verbose. In actual, `tname` is not used and `method` is always a C string literal in mruby-core.

In the future, we might probably define the following APIs by incompatible changes.

 * mrb_value mrb_check_convert_type(mrb_state *mrb, mrb_value val, enum mrb_vtype type, mrb_value tname, mrb_value method)
 * mrb_value
mrb_convert_type(mrb_state *mrb, mrb_value val, enum mrb_vtype type, mrb_value tname,  mrb_value method)