Partition map

[iDmitriyy]

MR is made as the result of the discussion with @natecook1000 : https://forums.swift.org/t/add-splitmap-function-to-swift-standard-library/69321

Checklist

• I've added at least one test that validates that my change is working, if appropriate
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary

[iDmitriyy]

One thing needed to discuss. As we decided to not make internal Either enum public, current names are PartitionMapResult2 and PartitionMapResult3.
While these types are made for concrete purpose, names are felt to be very long. My current thoughts are to use shorter names like PartitionMap2 and PartitionMap3, OneOf2Result / OneOf3Result or _OneOf2 / _OneOf3 .