Make the simplifier do let specialisation

[ollef]

If a function let binding is always called with the same argument, specialise it to that.

[ollef]

To expand on this, the idea is that if we have

let f = \x y z. foo x y z in bar (f a 1 b) (f c 1 d)

then we can specialise fs y parameter and get:

let f = \x z. foo x 1 z in bar (f a b) (f c d)

This can help e.g. if a let binding has its type generalised and is then used repeatedly with the same typeclass dictionary.

Some care has to be taken with termination, to not turn a lambda into something that is potentially non-terminating.

This might be a good issue to look at for someone who wants to dive into the compiler internals, since the scope of it is limited.