Added join-let and join-apply; bugfixes

[AdUhTkJm]

Join-let and join-apply refers to this control flow:

fn f() {
  for {
    if a > 0 {
      return 2;
    }
  }
}

For a statement let x = f(), the frontend will rewrite it into something like:

join-let ret = fn () { return 2; }
let x = for {
  if a > 0 {
    join-apply ret()
  }
}

The semantics is that join-apply behaves exact the same as br in WASM; in other words, it's similar to break in a loop, but it can carry a value.

Bugfixes:

• a ^ b used to get compiled into and %1, a, b. Now it produces xor correctly.
• unsigned conversion (from u32 to u64/i64) now works. Bits 32-63 can be any value for an u32, so we must erase them.