Currently regexp2 can be quite slow when a MatchTimeout is
supplied (a micro-benchmark shows 3000ns compared to 45ns
when no timeout is supplied). This slowdown is caused by
repeated timeout checks which call time.Now().

The new approach introduces a fast but approximate clock that
is just an atomic variable updated by a goroutine once very
100ms. The new timeout check just compares this variable to
the precomputed deadline.

Removed "timeout check skip" mechanism since a timeout
check is now very cheap.

Added a simple micro-benchmark that compares the speed
of searching 100 byte text with and without a timeout.

Performance impact:
1. A micro-benchmark that looks for an "easy" regexp in a 100
   byte string goes from ~3000ns to ~45ns.
2. Chroma (syntax highlighter) speeds up from ~500ms to ~50ms
   on a 24KB source file.
3. A background CPU load of ~0.15% is present until the end of
   of all match deadlines (even for matches that have finished).