Split OsString into separate package

[hasufell]

Since #202 introduces significant new API that really doesn't belong into filepath anymore, I want to split out the packages

This means GHC will need a new boot package os-string: https://github.com/haskell/os-string

@bgamari @mpickering @Bodigrim

[Bodigrim]

I don't have bandwidth to look into details, but the split sounds good to me.

[hasufell]

I don't have bandwidth to look into details, but the split sounds good to me.

The caveat is this: if a user adds both filepath and os-string to their dependencies they may get compile error if using modules that exist in both, due to deprecation period. That can easily be fixed with PackageImports. But it isn't a breaking change, technically.

In order to push people into using os-string, I'd prefer to not release the bytestring API in filepath.

[hasufell]

It also means: after the deprecation period, we'll have to do a major PVP bump to filepath, incurring some maintenance cost to stackage etc.

[hasufell]

The other options is to rename all module prefixes System.OsString to Data.OsString in os-string package. That way there won't be module name clash. But it's debatable which one is more annoying.