Add simple form of higher-order inlining

[myreen]

I suspect CakeML would do better on a number of benchmarks and real-world examples if we optimised tabulate, map, filter, genlist, every etc. for cases where they take a known closure as an argument and pass this closure to recursive calls unchanged.

Example:

fun map f [] = []
  | map f (x::xs) = (f x) :: map f xs
...
val xs = map (fn n => n+1) ys

should become (step 1):

fun map f [] = []
  | map f (x::xs) = (f x) :: map f xs
...
val xs = 
  (let 
     fun map f [] = []
       | map f (x::xs) = (f x) :: map f xs
   in map end) (fn n => n+1) ys

which becomes (step 2):

fun map f [] = []
  | map f (x::xs) = (f x) :: map f xs
...
val xs = 
  (let 
     fun map f [] = []
       | map f (x::xs) = ((fn n => n+1) x) :: map f xs
   in map end) 0 ys

which becomes (step 3):

fun map f [] = []
  | map f (x::xs) = (f x) :: map f xs
...
val xs = 
  (let 
     fun map f [] = []
       | map f (x::xs) = (let val n = x in n+1 end) :: map f xs
   in map end) 0 ys

Such an optimisation would benefit from coming after clos_known. In fact, the crucial steps 2-3 would follow by clos_call and various bvl optimisations, if the last f x produced by step 1 was annotated with the closure number of fn n => n+1.

@SOwens tells me that this paper is relevant, even though we would not aim at anything this comprehensive.

[myreen]

This has been done for PureCake: https://cakeml.org/esop24-inlining.pdf