change variant match function to use CallDefined?

[jckarter]

Instead of interleaving the instance types and lambdas:

match(variant, Inst1, i1 -> { .. }, Inst2, i2 -> { .. });

The match function would be cleaner if it dispatched matching based on the lambda signature:

match(variant, (i1:Inst1) -> { .. }, (i2:Inst2) -> { .. });

which would look nice with block syntax:

match(variant): (i1:Inst1) -> {
  ..
}, (i2:Inst2) -> {
  ..
}