Expose TarPath -> PosixString

[Bodigrim]

Essentially this, but with a better name:

tar/Codec/Archive/Tar/Types.hs

Lines 418 to 427 in 29ac5b5

	fromTarPathInternal :: PosixChar -> TarPath -> PosixString
	fromTarPathInternal sep = go
	where
	posixSep = PS.unsafeFromChar FilePath.Posix.pathSeparator
	adjustSeps = if sep == posixSep then id else
	PS.map $ \c -> if c == posixSep then sep else c
	go (TarPath name prefix)
	| PS.null prefix = adjustSeps name
	| PS.null name = adjustSeps prefix
	| otherwise = adjustSeps prefix <> PS.cons sep (adjustSeps name)

Or maybe just expose data TarPath from somewhere?..