[Issue 12379] Add toFile function which writes its first argument to a file

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=12379
Issue 12379 depends on issue 13009, which changed state.

Issue 13009 Summary: [REG2.064] inout overload conflicts with non-inout when used via alias this
https://issues.dlang.org/show_bug.cgi?id=13009

           What    |Removed                     |Added
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             Status|REOPENED                    |RESOLVED
         Resolution|---                         |FIXED

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