inferno-vc shouldn't use Manager internals

[ngua]

mkVCClientEnv in inferno-vc currently uses the internals of Manager (from http-client) to modify request headers and body:

inferno/inferno-vc/src/Inferno/VersionControl/Client.hs

Lines 19 to 43 in f229ad9

	mkVCClientEnv :: Manager -> BaseUrl -> ClientEnv
	mkVCClientEnv man@Manager {mModifyRequest = modReq} baseUrl =
	mkClientEnv man {mModifyRequest = modReq'} baseUrl
	where
	modReq' :: Request -> IO Request
	modReq' r = do
	x <- modReq r
	pure $
	if ((hContentEncoding, "gzip") `elem` requestHeaders x)
	then x
	else
	let new_hdrs = (hContentEncoding, "gzip") : requestHeaders x
	(hrds, body) = case requestBody x of
	RequestBodyBuilder _ _ -> (requestHeaders x, requestBody x)
	RequestBodyStream _ _ -> (requestHeaders x, requestBody x)
	RequestBodyStreamChunked _ -> (requestHeaders x, requestBody x)
	b -> (new_hdrs, compressBody b)
	in x {requestHeaders = hrds, requestBody = body}
	compressBody :: RequestBody -> RequestBody
	compressBody = \case
	RequestBodyLBS bsl -> RequestBodyLBS $ compress bsl
	RequestBodyBS bs -> RequestBodyLBS $ compress $ BSL.fromStrict bs
	RequestBodyIO iob -> RequestBodyIO $ compressBody <$> iob
	b -> b

I don't think we should rely on the internal implementation of Manager like this; the current docs for that Network.HTTP.Client.Internal state

No API stability is guaranteed for this module

which is generally the case for modules marked as internal.

We could probably just use a middleware to achieve the same thing. Or, if that's not possible, this code should document why it's necessary to use internal modules from another library