Improve gas efficiency of EnumerableMap

[Amxx]

EnumerableSet:

• use of inline assembly to optimise add
• use of inline assembly to optimise remove

EnumerableMap:

• Using an EnumerableSet for keys management + simple key => value mapping
• Optimizes for key lookup rather then index lookup

Gas report
Note: gas report doesn't look to be such a good indication. tests on remix show the assembly to be more efficient, but its difficult to quantify.

Function	Min	Max	Average
EnumerableSetMock.add	23630 → 23630	85403 → 85403	65740 → 65740
EnumerableSetMock.remove	22460 → 21450	26666 → 25656	25193 → 24471
EnumerableMapMock.set	25809 → 25038	105902 → 106023	87441 → 87400
EnumerableMapMock.remove	23592 → 24023	32502 → 28229	29053 → 26577

PR Checklist

• Tests
• Changelog entry

[frangio]

I have concerns about the use of assembly but the changes to EnumerableMap look good.

[frangio]

This is good but needs a changelog.

[frangio]

Looks good. Thank you!