Switch syntax sugar

[emil14]

See #724 for context. These forms should be equivalent:

Before (desugared)

def Foo(n int) (n int) {
    Switch<int>
    Bar, Baz, Bax
    ---
    :n -> switch:data
    1 -> switch:case[0] -> Bar
    2 -> switch:case[1] -> Baz
    switch:else -> Bax
    [bar, baz, bax] -> :n
}

After (sugar)

Given #726 (comment) switch will probably look like this:

v -> switch {
    sender_1 -> receiver_side_1
    sender_2 -> receiver_side_2
    ...
    default -> receiver_side_N
}