Reconsider internal representation of `FsPath`s

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 `OsPath`s/`FilePath`s. `FilePath`s would, however, probably not be more performant than the current `[Text]` representation.

Start by examining the current FsPath representation as [Text] and its combinator and conversion functions. Compare possible OsPath, FilePath, and Vector representations, including the invariants needed for sensible relative paths. Done means selecting and implementing a justified internal representation while simplifying the affected FsPath operations.

Written by the indexing model from the issue text.