Handle instantiation of unmanaged class via object literal

[dcodeIO]

As reported in #1183 instantiating an unmanaged class from an object literal, like in

@unmanaged
class Foo {
  a: i32;
}

var foo: Foo = { a: 1 };

results in unexpected behavior. Wasn't able to reproduce the issue Wasm Studio produces on current master, but it was indeed trying to retain/release unmanaged memory, which might have been the cause and should be fixed now, along a proper diagnostic when attempting this in --noUnsafe contexts. @jtenner Does this fix the original problem?

[jtenner]

It looks like it! @DuncanUszkay1 should review this as well.

[jtenner]

Also, this is going to make instantiating iovecs a bit more ergonomic.

[dcodeIO]

Would be great if someone with that issue could validate that the PR indeed fixes the problem :)

[jtenner]

I yeah tried out Duncan's example and it does work.

[jtenner]

Any update?