Correctly handle OsString

[Qwaz]

Currently, Soma mixes OsString and String in its internal code. This prevents using non-unicode file name even though host OS supports it.

Edit: non-UTF-8 to non-unicode

[KSAlpha]

Soma implementations reject non-unicode file name only when building docker images.

As non-unicode filename is a very rare use case, I would like to mark this as wontfix. Also, it seems to be a very hard problem to fix.

Supporting non-unicode filename is hard to address because:

• Soma supports both Windows and Linux systems. (i.e. unix OsStr is not reachable from Windows host)
• Text encoding of soma.toml may occur non-unicode texts to be lost (even before Soma handles it).

However, we would like to provide some workarounds later by supporting user-defined script, which will be executed inside the container in build time. And, by renaming some files using the script, non-unicode filenames can be present inside the image.