Add EnumerateRunes() ref APIs and unit tests

[GrabYourPitchforks]

Reference assembly APIs and unit tests for dotnet/coreclr#21007.
This PR will not pass CI until the coreclr PR is committed and the build system injects the new drop into corefx.

[tarekgh]

public partial struct RuneEnumerator [](start = 8, length = 36)

Talked offline with Levi if this can be moved outside String class. I am no ta fan of nested types but I will be ok if decided to keep it inside.

[GrabYourPitchforks]

The next logical place would be System.Text.StringRuneEnumerator. Since very few people will ever use this type directly (most should access it via a foreach, which hides the type name) I'm fine with moving it out in order to keep the string class cleaner.

[ahsonkhan]

nit: extra space

[GrabYourPitchforks]

It turns out we may have to move the nested class out to a standalone non-nested class anyway. System.String has a [TypeForwardedFrom(...)] attribute on it, which seems also to be applied to nested classes. This is causing unit test failures in the reflection layer. Should make @tarekgh happy to see it moved out. :)

[qbit86]

StringRuneEnumerator implements both IEnumerator<Rune> and IEnumerable<Rune> (with GetEnumerator() => this). Does not this mean that “foreaching” over it is not idempotent? Are there in BCL any other examples of public stateful enumerators that are “one-time self-enumerable”?

[stephentoub]

It's a struct. That makes a copy.

[qbit86]

@stephentoub
I like the idea and consider using this approach in my libraries. But what are the actual guidelines for implementing IEnumerable<T> by enumerator? FDGs say:

X DO NOT use IEnumerator<T>, IEnumerator, or any other type that implements either of these interfaces, except as the return type of a GetEnumerator method.
X DO NOT implement both IEnumerator<T> and IEnumerable<T> on the same type.

As far as I can see, here are only three public types in corefx that utilize GetEnumerator() => this trick: StringRuneEnumerator, SpanRuneEnumerator, and BlobBuilder.Blobs.