Slight performance boost on PickByKeys / OmitByKeys

[ericleb010]

Using map indexing, we can improve the performance of PickByKeys / OmitByKeys, by ranging over the input key array and checking the input map, rather than ranging over the input map and then ranging over the input key array.

I'm assuming that in most applications, the size of the input map is larger than the size of the input key array, so this should make things faster in practice.