Consider making `Drain` an enum, or using `Option::<Drain>::None` to represent no drain output(s).

[evanlinjin]

Relevant Context:

• Implement lowest fee metric correctly #13 (comment)

[LLFourn]

90% of the time you will just .unwrap_or(0) on the values so I came to the conclusion that in this case it's better just to let 0 signify None. I could be convinced otherwise but the comment doesn't weigh putting it in an Option that against the cost of matching or copious .unwrap_or(0).