Performance Improvements for Map

I maintain a fork of this repo with just Immutable Map at https://github.com/nkzw-tech/immutable-map published as @nkzw/immutable-map. I rebased on top of Immutable.js 5.x and had Codex 5.5 spend 13 hours on perf improvements. The results are here: https://github.com/nkzw-tech/immutable-map/commit/27d8255e5a836504435ffa1e172a41680560d858

It speeds up algorithms in a real world application such as Athena Crisis by roughly 2-5%. I figured I'll share the improvements back in case you want to review and adopt them as well!