AssemblyScript: Investigate using as-containers Option<T> class instead of Nullable<T>

[dOrgJelli]

More info here: https://github.com/yjhmelody/as-container#option

We're currently using the Result<T, E> class and it's working great.

We should see how this changes the developer experience, and if it's beneficial to us in some way over using Nullable<T>.

[dOrgJelli]

This work has been completed here:
#960
#944