Implement yield-from in Python 2.x as a macro

  And who said you can't teach an old dog new tricks.
Hello, I read about this PR in the dustycloud [blogpost](http://dustycloud.org/blog/how-hy-backported-yield-from-to-python2/). 

The idea of using macros to emulate `yield from` sounds wonderful. However, the implementation doesn't handle some edge cases described in the *Formal semantics* section of [PEP-380](https://www.python.org/dev/peps/pep-0380#formal-semantics). Was this deliberate or is it just a proof-of-concept at the moment?