Function for concatenating rather than replacing

[davidspies]

In the case where the variable is being accumulated by concatenating on a shrinking diff, it may be more efficient to skip the negate-and-then-un-negate part.

[frankmcsherry]

Sorry for the delay.

Talking this out, this looks like the SemigroupVariable, but is perhaps appropriate for folks who don't know statically that they will have the structural guarantee it provides. If that is the intent, it would be great to put some comments in to that effect (the comments in the file could generally be improved, which is on me). In particular, it would be great to say what set_concat corresponds to (self.set(self.concat(result), I think) and why one would use it (performance).

[frankmcsherry]

Going to merge a fast-follow with some refreshed documentation (re-reading it myself it now seems very problematic).