[Issue 13717] `split` no longer defined by std.string

[via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=13717

--- Comment #11 from hst...@quickfur.ath.cx ---
Also, @monarchdodra, if we're going to keep these functions in std.string, then
they need to be documented, instead of being "implicitly" defined.

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