support await in pattern guards

[retronym]

If possible.

In the meantime, I'll prohibited them explicitly.

[retronym]

Some brainstorming:

(42: Int) match {
  case P1 if G1 => E1 // assume that G1 suspends, G2 does not.
  case P2 if G2 => E2
  case P3 => E3
}

// translate to:

val temp = foo

def match1() = {
  temp match {
    case P1 =>
      if (G1) E1
      else match2()
    case _ =>
      match2()
  }

def match2() = {
  temp match {
    case P2 if G2 => E2
    case _ => match3()
  }
}

def match3() = {
  temp match {
    case P3 => P4
  }
}

match1()

// But, we lose exhaustiveness/unreachability analysis by doing this,
// not to mention common sub-pattern elimination.

[retronym]

Prototype of this idea: master...retronym:ticket/26