This commit adds the `ScopedFile` type, which wraps and derefs to a `File`. When
a `ScopedFile` is dropped, it removes the underlying file from the
filesystem. This is useful for temporary, generated files in tests.
Due to canonicalize()'s use of GetFinalPathNameByHandleW() on Windows,
the resolved path starts with '\\?\' to extend the limit of a given path
to 32,767 wide characters.
To address this issue, we remove this prepended string if available.
