Improve mapping over keys for IntMap and IntSet

[meooow25]

• Implement the functions in terms of the builders instead of converting to and from a list. This reduces time and allocations.
• Document that they take linear time for monotonic functions.

Closes #1027

---

Benchmarks with GHC 9.10.1:

IntSet:

Name            Time - - - - - - - -    Allocated - - - - -
                     A       B     %         A       B     %
map:asc          36 μs   22 μs  -37%    456 KB  136 KB  -70%
map:random      547 μs  513 μs   -6%    2.9 MB  2.6 MB  -10%
mapMonotonic     35 μs   23 μs  -34%    456 KB  136 KB  -70%

IntMap:

Name                    Time - - - - - - - -    Allocated - - - - -
                             A       B     %         A       B     %
mapKeys:asc             101 μs   68 μs  -32%    960 KB  608 KB  -36%
mapKeys:random          594 μs  600 μs   +1%    3.0 MB  2.7 MB  -11%
mapKeysMonotonic         94 μs   63 μs  -32%    960 KB  608 KB  -36%
mapKeysWith:asc:dups    129 μs   61 μs  -52%    1.2 MB  512 KB  -58%

[treeowl]

Good idea.

[meooow25]

Looks like we can get faster (~15%) if we work with the non-empty part of the builder separately, since we have, e.g.

data MonoState
  = MSNada
  | MSPush {-# UNPACK #-} !Int {-# UNPACK #-} !BitMap !Stack

If we get to first entry separately to avoid the MSNada, the MSPush can be unpacked in the rest of the fold. This isn't something GHC can figure out, though it is a bit like SpecConstr.

The downside is that we'll need a few more functions operating on non-empty MonoState, but it shouldn't be too messy.

[meooow25]

It turns out that SpecConstr can do it! This is really neat. The problem here is that foldlWithKey'

containers/containers/src/Data/IntMap/Internal.hs

Lines 3168 to 3179 in 316cde6

	foldlWithKey' :: (a -> Key -> b -> a) -> a -> IntMap b -> a
	foldlWithKey' f z = \t -> -- Use lambda t to be inlinable with two arguments only.
	case t of
	Bin p l r
	| signBranch p -> go (go z r) l -- put negative numbers before
	| otherwise -> go (go z l) r
	_ -> go z t
	where
	go !z' Nil = z'
	go z' (Tip kx x) = f z' kx x
	go z' (Bin _ l r) = go (go z' l) r
	{-# INLINE foldlWithKey' #-}

has go z' Nil = z' which disables it, probably to preserve the object.

But Nil can only occur at the top-level, so we can replace that RHS with error and move the Nil check to the top-level, which does the trick.