regex: expose lower level search APIs

[BurntSushi]

This commit exposes two new areas of API surface:

1. A new captures_read method which provides a way to access the
   offsets of submatches while amortizing the allocation of the
   space required to store those offsets. Callers should still of
   course prefer to use the higher level captures method, but if
   performance dictates, this lower level API may be useful.
2. New "at" variants of
   shortest_match/is_match/find/captures/captures_read that permit
   controlling where the start of a search begins within a slice.
   This is typically useful for controlling the match semantics of
   look-around operators such as ^ and $, and are necessary for
   implementing non-overlapping iterators.

Fixes #219

[BurntSushi]

cc @raphlinus I think you mentioned looking for these APIs at one point in the long ago. These have been available as undocumented APIs for quite a while, but now I actually need them for another project and I think they've mostly served me well.

[BurntSushi]

I've been using these new APIs for a bit now and everything feels OK to me. So I'm going to bring this in and push out a new release.