Fix: check computed members against listing/mapping owner

[bioball]

This addresses a regression where a value can possibly not be type-casted correctly.

Closes #785

[odenix]

Only executing owner's and receiver's type cast is insufficient. New test:

local a = new Listing { new Foo {} }
local b = (a) { new Bar {} }
local c = b as Listing<Bar>
local d = (c) { new Foo {} }
local e = d as Listing<Foo>

result = e

open class Foo
class Bar extends Foo

The easiest fix is to switch to my implementation. :-)

[bioball]

Ah, yup, you're right! I see what you mean now by it being insufficient to just check the owner and receiver.

I'll take a closer look at your PR.