compiler: improve logic when deciding between conjunctions and multi/multi_a #657
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There is more to come, but do a bunch now because it's cathartic.
When you grep the codebase for FIXME it's nice if the output isn't overwhelmed by the same message repeated 20 times.
This matches the same variant in Terminal, matches the string serialization "thresh", and will avoid weird compiler errors related to collisions with the upcoming `Threshold` type. This will be an annoying API-breaking change, but fortunately(?) this PR is already breaking the API for thresholds in an annoying way.
In our threshold logic we have some special cases to handle multi, multi_a, and conjunctions. However, we were not choosing between these special cases correctly. Our logic was roughly that we would try to use multi_a if all children were pk()s, OR try to use multi if all children were pk() and there weren't too many, OR try to use a conjunction if k == n. The correct logic is: if all children are keys, and there aren't too many, try to use multi or multi_a. ALSO, if k == n, try to use a conjunction. With this fix, the compiler correctly finds that conjunctions are more efficient than CHECKMULTISIG when k == n and n < 3. When n == 3 the two cases have equal cost, but it seems to prefer the conjunction. It also correctly finds that when k == n, it is always more efficient to use a conjunction than to use CHECKSIGADD. This change necessitates changing some tests.
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The compiler logic when encountering thresholds of pks is currently a bit confused, and therefore chooses multi/multi_a in cases where this is not the most efficient compilation.
This PR fixes that, and also brings in a few other cleanup commits that I had laying around.
This does not fix #656, which I didn't know how to approach.