Reduce String/ByteString/PosixString variants

[hasufell]

We currently have three variants of many modules. I think we should reduce these.

My idea is:

1. String is questionable overall, maybe drop it completely in unix-3.x.x.x
2. merge ByteString and PosixString... use PosixString for filepaths and short data (e.g. GroupEntry/UserEntry maybe) and use ByteString for file contents, lazy IO and other things where we don't know anything about size limits. RawFilePath should be dropped... it's not right.

So, ideally... there would be only one API. String will probably have to live much longer, but it could be debated to move it to a compat package some time in the future.

@hs-viktor @Bodigrim

[Bodigrim]

I do not expect another major version bump after unix-2.8 within a year or two. I'd just wait and see how things unfold wrt string zoo in Haskell and return to this discussion later.

[NicolasT]

Couldn't Backpack help here?

[hasufell]

Couldn't Backpack help here?

I think no one uses that.

[NicolasT]

Couldn't Backpack help here?

I think no one uses that.

Which is a bummer, because it's exactly the kind of functionality (ML-style modules and functors) that would reduce duplication 😄

It looks like Stack and some other non-Cabal build tools don't support it though, hence I guess it's ruled out for a library like unix. Which is a bummer. I may toy with a PoC, just to get familiar with Backpack.

[hasufell]

String discussions seem to be currently quite popular, so we could try to source community opinions.