Provide instrumentation points called "hooks".

Change-Id: I854b2ba264d8c7239efb8a80bc440b1f5e0955f5
Reviewed-on: https://code-review.googlesource.com/c/re2/+/51834
Reviewed-by: Randall Bosetti <rlb@google.com>
Reviewed-by: Paul Wankadia <junyer@google.com>
4 files changed