Proper tail calls in Rust FFI

[raviqqe]

Problem

• Tail calls are not eliminated in Rust FFI.
  • At least, not guaranteed...

Solution

• Options
  • Trampoline?
    • We do not return stacks because they are visible from callers.
    • e.g. return (continue_function, value);
  • Return values
    • This requires Rust functions to be idempotent.
    • e.g. return value;

Reference

• This should fix Stack overflow in life game example on x86_64 #659.
• Reviving tail-call elimination rust-lang/rfcs#2691

[raviqqe]

It turned out that we sometimes use trampoline functions even in the current implementation on suspension of futures in Rust by calling AsyncStack::suspend() although it would be better if we do this always without relying on nondeterministic suspension.