Make std.file.rmdir safe

It contains the following unsafe operations but we can verify that they can be trusted.

On Windows systems:
 - use of unsafe functions `core.sys.windows.windows.RemoveDirectoryW` and `std.internal.cstring.tempCStringW`

On Posix systems:
- use of unsafe functions `core.sys.posix.unistd.rmdir` and `std.internal.cstring.tempCString`

Additionally, I replaced `cenforce` with `wenforce` on Windows systems and changed the `Throws` section in the document.