Add inPlaceFM function

inPlaceFM descends a map and updates elements as it goes down based on keys.
This might be useful when you want to alter a lot of elements but don't want
to be forced to recurse the tree more than once.
Add applyMutationsInPlace and applyMutationsInPlaceIns functions

Uses FiniteMaps' inPlaceFM function to apply mutations in batch mode.