fix(frontend): crashes on fn ptr

[W95Psp]

This commit fixes the issue #757: the frontend was assuming functions expressions to be of a very specific shape. This commit fixes that.

[franziskuskiefer]

Thanks.

It looks like the following two functions get translated very differently. Is that on purpose?

fn do_twice(f: fn(i32) -> i32, arg: i32) -> i32 {
    f(arg) + f(arg)
}

fn do_again<F>(f: F, arg: i32) -> i32
where
    F: Fn(i32) -> i32,
{
    f(arg) + f(arg)
}

let do_again
      (#v_F: Type0)
      (#[FStar.Tactics.Typeclasses.tcresolve ()] i1: Core.Ops.Function.t_Fn v_F i32)
      (f: v_F)
      (arg: i32)
    : i32 =
  (Core.Ops.Function.f_call #v_F #i32 f (arg <: i32) <: i32) +!
  (Core.Ops.Function.f_call #v_F #i32 f (arg <: i32) <: i32)

let do_twice (f: (i32 -> i32)) (arg: i32) : i32 = f arg +! f arg

[W95Psp]

Yes, higher-order functions are a bit problematic, that's related to what @paulmure says in #549 (comment)