Optimize mypy.fastparse (Python 3 only) (#5722)

This implements various small optimizations to make AST conversion
from the typed_ast AST faster in compiled mypy. This makes compiled
mypy about 4% faster on self-check, and it also makes non-compiled
mypy around 1% faster on self-check.

Notable included optimizations:

* Implement visit methods more efficiently. Don't inherit visitors from
  typed_ast visitor classes.
* Refactor decorated methods away, as they are slow with mypyc.
* Refactor away many non-native attribute lookups.
* Refactor away some nested functions as they are slow with mypyc.

Also removed some dead code.

I'll implement the same optimizations for Python 2 AST conversions in
a separate PR.

Author: JukkaL