add eq-type

[vekatze]

Example:

define-meta sample<a>(): 'int {
  let b = magic eq-type(list(int), a);
  if b {
    quote {10}
  } else {
    quote {20}
  }
}

define use-eq-type(): unit {
  let _ = sample<list(int)>::(); // → 10
  let _ = sample<bool>::();      // → 20
  Unit
}