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

Command/Config.hs

@@ -55,23 +55,23 @@ optParser _ = setconfig <|> getconfig <|> unsetconfig
 
 seek :: Action -> CommandSeek
 seek (SetConfig ck@(ConfigKey name) val) = checkIsGlobalConfig ck $ commandAction $
-	startingUsualMessages (decodeBS' name) ai si $ do
+	startingUsualMessages (decodeBS name) ai si $ do
 		setGlobalConfig ck val
 		when (needLocalUpdate ck) $
 			setConfig ck (fromConfigValue val)
 		next $ return True
   where
 	ai = ActionItemOther (Just (fromConfigValue val))
-	si = SeekInput [decodeBS' name]
+	si = SeekInput [decodeBS name]
 seek (UnsetConfig ck@(ConfigKey name)) = checkIsGlobalConfig ck $ commandAction $
-	startingUsualMessages (decodeBS' name) ai si $ do
+	startingUsualMessages (decodeBS name) ai si $ do
 		unsetGlobalConfig ck
 		when (needLocalUpdate ck) $
 			unsetConfig ck
 		next $ return True
   where
 	ai = ActionItemOther (Just "unset")
-	si = SeekInput [decodeBS' name]
+	si = SeekInput [decodeBS name]
 seek (GetConfig ck) = checkIsGlobalConfig ck $ commandAction $
 	startingCustomOutput ai $ do
 		getGlobalConfig ck >>= \case