Finish implementing unpythonic.fix.fix

[Technologicat]

A.k.a. the function that detects and breaks recursion cycles. Related to Haskell's fix, but not the same thing.

Minimal TODO for a release-worthy version:

• DONE: Allow kwargs for f, everything else does.
• DONE: Make the implementation thread-safe, almost everything else is.
• DONE: Move unit tests to a new separate module unpythonic/test/test_fix.py.
• Investigate (but not necessarily resolve yet) whether we can reorganize things so we could guarantee to call bottom at most once. (Currently it's at least once, at most twice.)
• Document it in README, including credits for the original idea (Matthew Might) and initial Python implementation (Per Vognsen).

[Technologicat]

Unit tests moved in 09e1b5f. Thread-safety added in 3edb835.

[Technologicat]

Oops, didn't mean to close yet. Kwargs support added in 470a06b.

So the last thing for this issue, for now, is to write documentation for the README.

[Technologicat]

• DONE in 9ddd466: Fix the docstring, it currently says fix is not thread-safe, which is no longer true.
• DONE in 8bb8fd9 (though only visited needed it): Unwind-protect the changes to visited and cache by using a finally block. Currently, if f raises, garbage is left in the state.

[Technologicat]

Since this is now just waiting for a README update, which is tracked in #8, this separate issue is no longer needed.