Run to breakpoints when possible during async-signal replay instead of (always) single-stepping.

Makes degenerate cases run ~5-6x faster, and the unit test suite as a whole is ~1.25x faster, which was a nice surprise.  Resolves #233.

Still work to do on #218; replay is *wayyy* faster than before but still too high with default timeslice.  The idea in #231 should knock down the overhead some more, but I'm not sure by how much.  I.e. I don't know if it's slow syscallbuf-flushing or slow advance-to-exec-point that's dominating now.