Improve the Monoid.sumOption to avoid stack overflow

[johnynek]

Someone hit an issue making a giant list of TypedPipes with a stack overflow. This can be avoided by building a tree of merges rather than a big linear list (which also makes the depth of the graph lower, which generally should be a good thing).

This test was red when I started.

We could try to find other mitigations as well, but fundamentally we do stack-unsafe recursion when optimizing. Maybe it wouldn't kill us to use something like cats Eval to make it safe, but generally there is a big penalty for things like that.

[johnynek]

@ianoc take a look?

[ianoc]

lgtm