py/objstr: modulo_format: Add fast path for "%s" % "str" and b"%s" % b"bts".
py/objstr: modulo_format: Add fast path for "%s" % "str" and b"%s" % b"bts".

How %s was implemented is effectively by calling __str__() method on
argument and putting result into separate vstr_t, then applying field
width, etc, format specifiers to the result. But that's rather
inefficient when we want to format string into string (or bytes into
bytes). Add a fast path for these cases, avoiding extra vstr_t allocation.
After this, it's effectively equivalent to how "%d" is handled for int's
(it's formatted directly into the result vstr_t, without any intermediate
__str__ and vstr_t).

pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython