Support subscript setters

[harlanhaskins]

Subscripts are currently get-only, which means you can't override the setting behavior.

Something like this should work

type Foo {
  subscript(_ v: Int) -> Int {
    get { return 4 }
    set { print("you gave me"); println(newValue) }
  }
}

[segiddins]

Should a SubscriptSetExpr return anything or just void? (I personally like setters returning the set value)

[harlanhaskins]

I actually think setters should be void