[Issue 18239] std.experimental.allocator fillWithMemcpy could use memset when T.sizeof==1

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=18239

--- Comment #2 from Nathan S. <n8sh.secondary at hotmail.com> ---
(In reply to Nathan S. from comment #1)
> Could also use wmemset when T.sizeof == wint_t.sizeof

Unfortunately core.stdc.wchar_.wmemset isn't marked as pure so this would cause
some tests to fail.

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