[PEAK] Trellis on_commit and Performers

[Sergey Schetinin]

The first suggested solution (with maintain and was_set) seems to work
better, at least there's much less head-scratching so far. I kinda
knew all along that non-lazy event binding would make it simpler, but
there are so many events that exist and thus will be subscribed to,
just because the library supports them, that I still want to do it
properly. I'll see if I can poke any holes in this maintain / was_set
implementation. Thanks for the suggestion!