This patch simplifies the str creation API to favour the common case of
creating a str object that is not forced to be interned.  To force
interning of a new str the new mp_obj_new_str_via_qstr function is added,
and should only be used if warranted.

Apart from simplifying the mp_obj_new_str function (and making it have the
same signature as mp_obj_new_bytes), this patch also reduces code size by a
bit (-16 bytes for bare-arm and roughly -40 bytes on the bare-metal archs).