core: use specialization for more methods of impl Iterator for &mut I
#138862
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...and get rid of
ByRefSized. Most of the iterator methods can be forwarded to the inner iterator if we know that it isSized, but this is currently only done forfoldandtry_fold. This PR adds the optimization for all applicable methods. It uses two private helper struct to hide the specialization from the public API:Iterator for ByRefDefaultonly forwards the methods that can be used even when the iterator is notSized, all other methods are left to the default.Iterator for ByRefspecializes onSizedand forwards to either the underlying iterator or toByRefDefaultThis PR also removes
ByRefSized, theIterator for &mut Ishould now be good enough to be used everywhere.