implement varargs

[Gankra]

Strawman varargs syntax for saying there's varargs but that you want to test this particular set of values passed in them:

func "blah" {
    inputs {
        x "u32"
        "..." {
            _ "f32"
            _ "f32"
        }
    }
}

SUB TODO: figure out if varargs should allow for specifying what the callee and caller think is happening as different

SUB TODO: figure out if we should try to support that fucked up thing where Swift supports multiple named varargs lists

[Gankra]

cc https://github.com/Gankra/abi-cafe/issues/1