`indexOf` allocates `Maybe`s

[sjakobi]

unordered-containers/Data/HashMap/Internal.hs

Lines 2292 to 2303 in 59ddae5

	-- | \(O(n)\) Lookup the value associated with the given key in this
	-- array. Returns 'Nothing' if the key wasn't found.
	indexOf :: Eq k => k -> A.Array (Leaf k v) -> Maybe Int
	indexOf k0 ary0 = go k0 ary0 0 (A.length ary0)
	where
	go !k !ary !i !n
	| i >= n = Nothing
	| otherwise = case A.index ary i of
	(L kx _)
	| k == kx -> Just i
	| otherwise -> go k ary (i+1) n
	{-# INLINABLE indexOf #-}

It probably doesn't matter much, but it would be more efficient to return -1 when the key isn't found.