[Issue 21026] add core.bitop.bswap(ushort)

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21026

--- Comment #2 from Walter Bright <bugzilla at digitalmars.com> ---
Because of overloading with bswap(uint), this breaks existing code. Rename to
byteswap(ushort).

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