JavaScript: Re-add optimisation for literal strings

In GHCJS, the genApp function contained an optimisation for unpacking string literals:

  -- special cases for JSString literals
  -- we could handle unpackNBytes# here, but that's probably not common
  -- enough to warrant a special case
  | [StgVarArg v] <- args
  , [top] <- concatMap snd (ctxTarget ctx)
  , matchVarName "ghcjs-prim" "GHCJS.Prim" "unsafeUnpackJSStringUtf8##" i =
     (,ExprInline Nothing) . (|=) top . app "h$decodeUtf8z" <$> varsForId v

This wasn't ported with the JavaScript backend, but should be added now in GHC.StgToJS.Apply.genApp.

---

This optimization too:

genApp ctx i [StgLitArg (LitString bs), x]
    | [top] <- concatMap snd (ctx ^. ctxTarget), getUnique i == unpackCStringAppendIdKey, Just d <- decodeModifiedUTF8 bs = do
        -- fixme breaks assumption in codegen if bs doesn't decode
        prof <- csProf <$> use gsSettings
        let profArg = if prof then [jCafCCS] else []
        a <- genArg x
        return (top |= app "h$appendToHsStringA" ([toJExpr d, toJExpr a] ++ profArg)
               ,ExprInline Nothing)