This commit fixes a bug where the parser could produce a very deeply
nested Hir value beyond the configured nested limit. This was caused by
the fact that the Hir can have some of its nested structures added to it
without a corresponding recursive call in the parser. For example,
repetition operators. This means that even if we don't blow the nest
limit in the parser, the Hir itself can still become nested beyond the
limit. This in turn will make it possible to unintentionally overflow
the stack in subsequent recursion over the Hir value, such as in the
Thompson NFA compiler.

We fix this by checking the nesting limit both on every recursive parse
call and also on the depth of the final Hir value once parsing is
finished but before it has returned to the caller.

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=60608

PR #1107

It turns out that we missed another case where the stack could overflow:
dropping a deeply nested Hir. Namely, since we permit deeply nested Hirs
to be constructed and only reject them after determining they are too
deeply nested, they still then need to be dropped. We fix this by
implementing a custom a Drop impl that uses the heap to traverse the Hir
and drop things without using unbounded stack space.

An alternative way to fix this would be to adjust the parser somehow to
avoid building deeply nested Hir values in the first place. But that
seems trickier, so we just stick with this for now.

This reverts commit 8a8d599 and
includes a regression test, as well as a tweak to a log message.

Essentially, the broadening was improper. We have to be careful when
dealing with suffixes as opposed to prefixes. Namely, my logic
previously was that the broadening was okay because we were already
doing it for the reverse inner optimization. But the reverse inner
optimization works with prefixes, not suffixes. So the comparison wasn't
quite correct.

This goes back to only applying the reverse suffix optimization when
there is a non-empty single common suffix.

Fixes #1110
Ref astral-sh/ruff#7980