Do we want to support `OsString`?

[supki]

So, it seems that using OsString in place of Strings can help to avoid some pretty pointless convertions (OsString -> String -> OsString) in the common case of putting filepaths into the environment, assuming people use OsString in the first place.

1. Do people actually use OsString?
2. Do they care that there are extra convertions?
3. This is going to be a breaking change (String is pretty much baked in everything right now)
4. Where is getEnvironment :: [(OsString, OsString)]? Obviously, we could conjure it up ourselves, but (looking at https://hackage.haskell.org/package/ghc-internal-9.1001.0/docs/src/GHC.Internal.System.Environment.html#getEnvironment) it doesn't sound fun at all. Also, I'd rather someone else maintained a Windows-compatibility layer.

Personally, I don't care that much about it, but it seems that OsString is going to be the way forward, so we'd have to support it eventually.