convert to `PosixPath`

[hasufell]

caveats:

• on windows we'll have to assume UTF-8
• we probably should wait for Add more bytestring like functions filepath#202 (or os-string package) to get cleaner access to certain functions

context: #6 (comment)

[Bodigrim]

I did as much work as possible without changing public API (and thus without a major version bump) in dfc8fc4 and c2c70ce. This would suffice to support Unicode filenames though.

[hasufell]

Yeah, but it doesn't support non-unicode on unix. I did not have time to look at anything. Feel free to rebase this on master.