Implement yield-to-yield from conversion

[colin99d]

A part of #827.

I am creating this draft pr for visibility. This is easily the most challenging fixer I have done, due to the infinite amount of nesting of functions inside functions allowed in python. Because of this I have created a recursive function to get all potential yield from statements. There is still a lot of work to be done, but I just wanted to push this in case anyone sees a much simpler way to do it.

[colin99d]

This is done and ready for review. It passes all the tests provided by pyupgrade except for one case I document in the code. Current issues are as follows:

1. Our implementation keeps more comments than pyupgrade does (this is the issue mentioned above). However, I think this is more of a plus than a minus. I REALLY doubt people want their comments removed by our linter.
2. The implementation would fail if before or after the loop x is defined as a separate thing and used. Our implementation would err in the side of NOT making changes.

[charliermarsh]

Will try to get to this tomorrow, thanks!

[charliermarsh]

@colin99d - I ended up refactoring this quite a bit. You might be interested in checking out the current implementation. The logic is roughly: do one pass over the function to find any candidate yield statements to convert, do a second pass to find any references to variables, then cross-reference to ensure we don't rewrite any yields whose variables are referenced elsewhere.