Extend Pattern Matching with Literals

[Jonathan Immanuel Brachthäuser]

Originally, we supported pattern matching on literals, such as

e match {
  case 1 => "one"
  case _ => "other"
}

but since we moved from matching combinators to a proper compilation of pattern matching, this feature was dropped. In the meeting, yesterday, @phischu brought up that we might be able to more easily support literal matchers by first supporting guards.

That is, first support

e match {
  case x if x == 1 => "one"
  case _ => "other"
}

which compiles to

let x = e;
if (x == 1) "one"
else "other"

or similar. And then desugar literal matches to guards.