Optimize unspecialized OrderedSet.init and OrderedSet.firstIndex(of:)

[dnadoba]

I have noticed that OrderedSet performance is slower than Dictionary if the Element isn't known at compile time and therefore the compiler can't specialize the implementations.

This PR replaces some of the generic functions that are generic with manual partial specialized variations that are only generic over the Element type.

Checklist

• I've read the Contribution Guidelines
• My contributions are licensed under the Swift license.
• I've followed the coding style of the rest of the project.
• I've added tests covering all new code paths my change adds to the project (if appropriate).
• I've added benchmarks covering new functionality (if appropriate).
• I've verified that my change does not break any existing tests or introduce unexplained benchmark regressions.
• I've updated the documentation if necessary.

[dnadoba]

This is a benchmark of the unspecialized version of OrderedSet.init and OrderedSet.firstIndex(of:) before and after this change

[dnadoba]

This is OrderedSet.init alone before and after