Reconsider internal representation of `FsPath`s

[jorisdral]

The current FsPath representation as [Text] is not very ergonomic. We should reconsider whether we want to pick a different internal representation, maybe something like an OsPath or Vector.

If we pick OsPath (or even FilePath) as the internal representation, with sufficient invariants to ensure that they form sensible relative paths, then we could probably simplify the implementation of a number of FsPath combinator and conversion functions. For example, (</>) on FsPaths could be a simple wrapper around (</>) on OsPaths/FilePaths. FilePaths would, however, probably not be more performant than the current [Text] representation.