Change Amt::for_each_mut to ignore unchanged values

[timvermeulen]

Summary of changes
Changes introduced in this pull request:

Adds an amt::ValueMut wrapper type that wraps a mutable reference to an element in the Amt and keeps track of whether it was mutated, to avoid flushing nodes later that weren't changed in the first place.

Also updates the BitFieldQueue to correctly only write a value back conditionally.

Reference issue to close (if applicable)

Closes #736

[austinabell]

Great use of the guard pattern! This seems generally correct, but I am definitely a bit worried about matching their caching for this. Checking the go implementation it seems like they aren't caching nodes through for_each on the Amt.

Anyways, that can be tackled and tested later, this should be good for now

[austinabell]

I'm pretty sure that for the exact usage there will still be a difference in blockstore interactions from the go implementation, but I guess this can be iteratively fixed. Would have been nice to add or modify a test to check this use