any function can be passed around as a normal value.
this requires us to create a wrapper around the function, which
converts the thin (normal) calling convention of the function into
the fat calling convention of a function containing some state.
this wrapper is called a "thunk".

<<<

add = fun (a int, b int) int { return a + b }

apply = fun (f fun (a int, b int) int, x int, y int) int {
    return f(x, y)
}

main = fun () {
    var r int = apply(add, 5, 7)
}

>>>

#include "core.c"
struct __cat_type_0 {
    void* state;
    integer (*call)(void* state, integer, integer);
};
integer add(integer a, integer b);
integer __cat_thunk_add(void* state, integer, integer);
integer apply(struct __cat_type_0 f, integer x, integer y);
void catskill_main(void);
integer add(integer a, integer b) {
    return a + b;
}
integer __cat_thunk_add(void* state, integer a, integer b) {
    (void)state;
    return add(a, b);
}
integer apply(struct __cat_type_0 f, integer x, integer y) {
    return f.call(f.state, x, y);
}
void catskill_main(void) {
    integer r = apply((struct __cat_type_0){ .state = nil, .call = __cat_thunk_add }, 5, 7);
}
#include "runtime.c"