Refactor `Reaction`

[kalekundert]

Reaction should be a list-like data structure under the hood. That would make it much easier to manipulate the reagents without having to contort myself to keep them in order. I'll need a dictionary too, because the O(1) lookups it provides are very useful, but the list should be the "single source of truth". The dict should be kept in sync with the list.

[kalekundert]

Also, should have ways to copy reagents between reactions. Useful for splitting reactions apart, which comes up not-infrequently.