Implement DoubleEndedIterator where possible

[bluss]

Use the DoubleEndedIterator abstraction where possible and reasonable in the libstd and libextra.

Particularly std::str has many iterators that can be made bidirectional, like StrBytesIterator, StrCharIterator, StrCharSplitIterator

[alexcrichton]

These implementations all exist today, closing.