Split wrappers

[davidism]

Closes #1450

Used https://github.com/potherca-bash/git-split-file to preserve the commit history so blame still works for each file. That's the reason for so many commits, it creates a branch to rename __init__.py to each split file.

werkzeug.wrappers still exports all the classes, the documentation remains the same. The understanding is that the documented API was the publicly importable API. However, none of the incidental imports are exposed at the top level any more, so anything that was behaving badly and importing werkzeug.wrappers.datetime, etc. will need to update.