Support defining and calling constant functions

[udoprog]

This adds support for defining and calling functions like these:

const VALUE = "baz";

const fn foo(n) {
    `foo {n}`
}
    
fn main() {
    foo(`bar {VALUE}`)
}

const fn's have the same restrictions as constant items, except that their input parameters can be defined.

We still need to add support for calling const fns from other const fns or const items.