Match all of a union

match.pony

```
primitive A
primitive B
primitive C
type ABC is (A | B | C)

actor Main
  var x: ABC = A
  var n: USize = 0
  new create( env: Env ) =>
    n = match x
        | A => 1
	| B => 2
 	| C => 3
	end
```

This gets the following error:
```
Error:
/home/pd/Develop/PonyLang/t/t.pony:10:7: right side must be a subtype of left side
    n = match x
      ^
    Info:
    /home/pd/Develop/PonyLang/ponyc/packages/builtin/none.pony:1:1: None val is not a subtype of USize val
    primitive None is Stringable
    ^
    /home/pd/Develop/PonyLang/ponyc/packages/builtin/none.pony:1:1: not every element of (None val | USize val) is a subtype of USize val
    primitive None is Stringable
    ^
```