Split string by string

[dfabulich]

This PR may be a bit surprising, because it removes func split(separator: Character) and adds in func split(separator: String). To my surprise, even when I left both overloads in, the existing character-based tests were all just using the string version once I added it.

Since the character-based implementation was, as far as I could tell, dead code, I eliminated it.

The implementation tries to use Kotlin split() if we're not omittingEmptySequences, but omittingEmptySequences defaults to true, so a simple "a,b,c".split(separator: ",") will not use Kotlin split().

(I also had to implement a whole separate code path for when the separator is empty, to match Swift's behavior in that case.)

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Cursor generated a first draft (especially the tests), and I rewrote its output.