Rename module to module_

[EricCousineau-TRI]

Avoids potential conflict with C++20 keyword

Resolves #1593
Seemed simple enough, just a quick pass using QtCreator and some follow-up passes. Doesn't update *.rst docs.

\cc @dalboris

[EricCousineau-TRI]

Seems like other PRs are suffering from the same issues, e.g. #1641.

@jagerman TravisCI failures just seem like a URL flake. Any chance you have an idea on the MSVC front?
(Regarding this PR, I'm fine letting it stagnate for a while, but just wanted to emphasize that other PRs may be blocked by this.)

[bstaletic]

CI is fixed, but some tests are flaky, so try rebasing onto master.

[EricCousineau-TRI]

Done - thanks!

[henryiii]

Looks like module is okay to keep: https://en.cppreference.com/w/cpp/keyword (it’s a special identifier).

[EricCousineau-TRI]

Sweet! Closing.