Replace ChildrenMap

[rkuris]

There is already a bitmap type used to serialize proof nodes:

firewood/firewood/src/proofs/childmap.rs

Line 34 in c4fb05f

pub(super) struct ChildrenMap([u8; ChildrenMap::SIZE]);

This one is better as that one was written before we removed branch_factor_256 so it was trying to be generic to support both. We should replace that one with this one.

Originally posted by @demosdemon in #1849 (comment)