Call generic_visit in all paths and bail out early in all visit methods

[lysnikolaou]

• Not calling generic_visit in all paths meant that we we missing some usages of thread-unsafe functions.
• Checking self.thread_unsafe in visit makes sure that we don't miss it if we add more visit_* methods in the future.

[ngoldbaum]

For all these PRs, would you mind doing a quick before/after on SciPy and Scikit-Learn like on the last PR?

It also might be more efficient for everyone if you just to one pull request instead of a series, since I think I'm going to ask you to do more or less the same checks on every PR.

[lysnikolaou]

Yeah, I'll be posting a before/after for all PRs.

I'm a bit hesitant to do one big PR since it's gonna be quite a bit harder to review. But I'm okay to close this and open one big one if you feel like it'll work better for you.

[ngoldbaum]

I can review it commit by commit, just try to keep the commit history nice.