[TS] Fix generation for traits that have supertraits

[TS] Fix generation for traits that have supertraits

When we have a trait with a supertrait, we have to pass both the
main- and super-trait instance indxes through to C so that it can
call both. We failed to do this, missing an argument entirely when
calling the C code.

Instead of trying to allocate a new instance index, we opt to store
instance indexes in trait structs (assuming there is one) and then
reuse the one in the constructed object.