test marker traits more rigorously on public API types

Since the regex crate uses interior mutability and a bit of synchronization, it follows that marker traits like Send, Sync, UnwindSafe and RefUnwindSafe are quite relevant and can change based on internal implementation details. #749 for example, removed the thread_local dependency and got a bit more rigorous about these marker traits on the main Regex/RegexSet types, but stopped there.

It turns out other types are impacted too. @kangalioo noted that, for example, Send was not implemented on the CaptureMatches iterator in regex 1.3.9, but it is now implemented in regex 1.4.5. I'm actually not sure why it wasn't implemented before. The regex crate doesn't use any non-Send stuff, so I'm not sure where that was coming from. I haven't investigated yet.

Either way, this is a compatibility hazard. Going from !Send to Send is fine, but it seems just as likely that we could regress too. The only way I can think to solve this is to assert expected marker traits for all public API types. It seems kind of absurd and tedious, but I don't have any better ideas.

Review issue #749 and the public API types named here, including Regex, RegexSet, and CaptureMatches. Determine the expected Send, Sync, UnwindSafe, and RefUnwindSafe behavior for all public API types. Done means marker-trait assertions cover the public API and prevent compatibility regressions.