[bindings] Concretize Result types without type aliasing

[bindings] Concretize Result types without type aliasing

While the type aliasing trick works great for cbindgen,
wasm_bindgen doesn't support it and requires fully-concrete types.
In order to better support wasm_bindgen in the future, we do so
here, adding a function which manually writes out almost the exact
thing which was templated previously in concrete form.

As a nice side-effect, we no longer have to allocate and free a u8
for generic parameters which were `()` (though we still do in some
conversion functions, which we can get rid of when we similarly
concretize all generics fully).