The unix package provides RawFilePath which is a type synonym of
ByteString. Unlike FilePath (which is String), it has no performance issues
because it is ByteString. It has no encoding issues because it is
ByteString which is a sequence of bytes instead of characters.

That's all good. With RawFilePath, we can properly separate the "sequence
of bytes" and the "sequence of Unicode characters." The control is
yours. Properly encode or decode them with UTF-8 or UTF-16 or any codec of
your choice.

However,

* The functions in unix are low-level.
* The higher-level packages such as process and directory are strictly tied
  to FilePath.

This library provides the higher-level interface with RawFilePath.