simplify and speed up Utility.FileSystemEncoding

This eliminates the distinction between decodeBS and decodeBS', encodeBS
and encodeBS', etc. The old implementation truncated at NUL, and the
primed versions had to do extra work to avoid that problem. The new
implementation does not truncate at NUL, and is also a lot faster.
(Benchmarked at 2x faster for decodeBS and 3x for encodeBS; more for the
primed versions.)

Note that filepath-bytestring 1.4.2.1.8 contains the same optimisation,
and upgrading to it will speed up to/fromRawFilePath.

AFAIK, nothing relied on the old behavior of truncating at NUL. Some
code used the faster versions in places where I was sure there would not
be a NUL. So this change is unlikely to break anything.

Also, moved s2w8 and w82s out of the module, as they do not involve
filesystem encoding really.

Sponsored-by: Shae Erisson on Patreon

Git/Ref.hs

@@ -82,7 +82,7 @@ branchFileRef branch f = Ref $ fromRef' branch <> ":" <> toInternalGitPath f
 
 {- Converts a Ref to refer to the content of the Ref on a given date. -}
 dateRef :: Ref -> RefDate -> Ref
-dateRef r (RefDate d) = Ref $ fromRef' r <> "@" <> encodeBS' d
+dateRef r (RefDate d) = Ref $ fromRef' r <> "@" <> encodeBS d
 
 {- A Ref that can be used to refer to a file in the repository as it
  - appears in a given Ref. -}
@@ -177,7 +177,7 @@ tree (Ref ref) = extractSha <$$> pipeReadStrict
 	[ Param "rev-parse"
 	, Param "--verify"
 	, Param "--quiet"
-	, Param (decodeBS' ref')
+	, Param (decodeBS ref')
 	]
   where
 	ref' = if ":" `S.isInfixOf` ref