Expose Regex memory usage info (fixes #943)

[ashdnazg]

Based on #1180

[BurntSushi]

So actually, thinking through this more, I think this routine is probably totally bunk. In particular, it does not account for the heap memory used by the lazy DFA, which is probably the biggest (ab)user of heap in most regexes.

It's non-trivial to get that heap usage because it's guarded behind a Pool, which specifically does not expose any way of telling how big it is. And doing so is not straight-forward since it's guarded by mutexes.

Unfortunately I think that if you really really need to know about the sizes of regex values, then you'd have to drop down to regex-automata and dead with Cache values directly.

Unfortunately, I just cannot think of a way to write documentation for this routine, as-is, that isn't some kind of outright lie in a meaningful sense.

[ashdnazg]

That makes sense, but it kinda means meta::Regex::memory_usage() is bunk as well.