Efficient signatures for patterns

[s5bug]

This needs an extra set of eyes and a bit of bug testing because I wrote all this while tired and hungry

Continuation of #1260, much thanks to @YukkuriC for the inspiration and motivation

[s5bug]

I would like to mention that due to object padding, HexSignature actually has room on it to store a cached Action. However, I don't know if it's safe to do that: I don't know where serialization boundaries are / what would happen if someone rerolled per-world patterns while a Signature had an old Action cached.

If it would be safe to do that, please let me know so I can implement it :)

[s5bug]

The other option (mentioned in Discord but not here) is to use that extra four bytes for int size; to not have to use TURNS_PER_ARRAY_ELEMENT * packedTurns.length. However, there's nothing that explicitly depends on the exact stroke count of a pattern to my knowledge.

[s5bug]

I don't have time to do it right now, but I need to remember to document properly that build can be called multiple times and reset can be used to not have to re-allocate.