GC, parent call and js.get

js.get always return the same table if the same js object/function is referenced
garbage collection work, so when a js.get'ed variable is collected in lua, the corresponding thing in the Lua.wrappers table will be freed too
parent object is tracked to make calls with a correct "this"
foo.bar = zog works, correctly assigning it on the JS side
when a lua callback is called, if "this" is in the lua store it gets passed as the first parameter (aka "self")