Handle lifetime tracking in GC targets

[shadaj]

For targets like JavaScript, we can introduce special references between diplomat objects that ensure they are kept around according to lifetime constraints.

For example, if we have

struct Foo(...)
struct Bar<'f>(...)

impl <'f> Bar<'f> {
  fn new(foo: &'f Foo) -> Bar<'f> {
    ...
  }
}

then the JavaScript type for Bar should have an additional field diplomat_lifetime_f, which is used to hold a reference to the Foo passed into the constructor

class Bar {
  static new(foo) {
    ...
    const out = new Bar(wasm.Bar_new(foo.underlying));
    out.diplomat_lifetime_f = foo;
    ...
    return out;
  }
}