Add the comptime keyword (with no effect yet)

[mbenke]

This PR introduces syntax - just syntax - for annotating parameters with comptime keyword, e.g.

contract ComptimeSyntax {

  function f(comptime x : word) -> comptime word {
    return x;
  }

  function g() -> word {
    let y : comptime word = f(42);
    return y;
  }

  function main() {
    return g();
  }
}

[rodrigogribeiro]

LGTM.