Add docstring
Refactor function type checking

Prepare for type checking type variables with values.
Fix None type handling in AST transform
Minor refactoring
Implement some type checking of tvars with values

Functions and a single type variable per function supported.
Add type checker test case
Fix functions with typevar values
Add type checker test case
Minor docstring update
Skip some if/else blocks that are not reached

Do not type check blocks that can't be reached due to an always-True or
always-False isinstance check.
Update typing.typevar to accept values kw argument
Add typevar values test case
Add typing.AnyStr
Add typing.AnyStr stub
Make file timestamps floats

All timestamps should be floats.
fnmatch: Take advantage of AnyStr
genericpath: Use AnyStr
Fix partial type application with typevar values
genericpath: Refactoring
posixpath: Refactor to use AnyStr