Make the map monoid more performant for mutable maps
fix up typo
avoid copy from mutable
Revert "avoid copy from mutable"

This reverts commit ed5a9425bc19c06cbd27041d52a2e30f17025468.
make an immtable backed map
remove benchmark
Merge pull request #245 from hansmire/develop

Make the map monoid more performant for mutable maps