add guard clauses #5

Introduce guard clauses as a syntax sugar, enabling early returns:

let safeDivide = fn (num, denom) -> {
  guard (denom != 0) else 0;

  num / denom
}

... would desugar to:

let safeDivide = fn (num, denom) -> {
  if (denom != 0) {
    num / denom 
  } else {
    0
  }
}