Previously the desired output type was specified. Now make the type part
of the function name. Because this function is used in a few places this
saves code size due to smaller call-site.
This makes `mp_obj_new_str_type_from_vstr` a private function of objstr.c
(which is almost the only place where the output type isn't a compile-time
constant).
This saves ~140 bytes on PYBV11.
Signed-off-by: Jim Mussared <jim.mussared@gmail.com>