Change signature of Iterator.size_hint

Remove the Option wrapper around the lower bound. None is semantically
the same as Size(0), so there's no point in having a distinction.
Implement .size_hint() on the remaining Iterator adaptors

Every iterator adaptor now has an implementation of .size_hint() that
makes sense, except for when the default of (0, None) is correct.
Implement size_hint() on Iterators in libextra