refactor: make Response module pure

[develop7]

Part of #2771

created per the suggestion at #2964 (comment)

[develop7]

@steve-chavez done

[steve-chavez]

I expected the Wai module usage to go away from Response after this change, but it's still in some places:

postgrest/src/PostgREST/Response.hs

Lines 284 to 290 in a6e3eda

	addRetryHint :: Int -> Wai.Response -> Wai.Response
	addRetryHint delay response = do
	let h = ("Retry-After", BS.pack $ show delay)
	Wai.mapResponseHeaders (\hs -> if isServiceUnavailable response then h:hs else hs) response
	isServiceUnavailable :: Wai.Response -> Bool
	isServiceUnavailable response = Wai.responseStatus response == HTTP.status503

postgrest/src/PostgREST/Response.hs

Lines 310 to 316 in a6e3eda

	traceHeaderMiddleware :: AppConfig -> Wai.Middleware
	traceHeaderMiddleware AppConfig{configServerTraceHeader} app req respond =
	case configServerTraceHeader of
	Nothing -> app req respond
	Just hdr ->
	let hdrVal = L.lookup hdr $ Wai.requestHeaders req in
	app req (respond . Wai.mapResponseHeaders ([(hdr, fromMaybe mempty hdrVal)] ++))

I don't see an easy fix for those now so LGTM.

[develop7]

I don't see an easy fix for those now so LGTM.

um, why, the rest of module doesn't depend on them, so they could reside somewhere in/under App just as well

[steve-chavez]

um, why, the rest of module doesn't depend on them, so they could reside somewhere in/under App just as well

Ah right, simply moving them to another module would be fine.