Implement preservation of comments "within" typing classes

This ensures that comments which appear around TypedDicts and
NamedTuples that are converted to class style are preserved.
The layout of the comments in the new code isn't perfect, however
that's relatively easy for the user to fix manually afterwards.

Fixes https://github.com/asottile/pyupgrade/issues/497
Fixes https://github.com/asottile/pyupgrade/issues/634
Fixes https://github.com/asottile/pyupgrade/issues/697