Replace `FilePath` by `OsPath`

[janmasrovira]

We are currently using the library filepath-1.4.2.2. In this version, the library offers its API for the type Filepath:

type FilePath = String

We are all aware of the problems of using String in haskell.

Recently, a newer version of the library, filepath-1.4.100.0, introduces a new type, called OsPath for encoding file paths in a more efficient and safe way. One can read the benefits of it in the documentation of the library.

I think we should refactor all uses of FilePath by OsPath.

[janmasrovira]

Adding the filepath-1.4.100.0 dependency causes dependency conflicts with other dependencies. I'll freeze this issue until filepath-1.4.100.0 is part of a stackage nightly

[lukaszcz]

We're using the path library.