Nullable Scalar Type Support

[dOrgJelli]

Currently, the only "nullable" types that are supported are objects, since AssemblyScrpit does not natively support "nullable scalar types" (ex: value: boolean | null).

In order to support nullable scalar types, a utility class Nullable<T> can be used. An example implementation: https://github.com/polywrap/monorepo/blob/prealpha/packages/wasm/as/assembly/msgpack/Nullable.ts

Example usage:

const nullable = Nullable.fromValue<u32>(5);

nullable.isNull; // false
nullable.value; // 5
nullable.value = 0;

[Niraj-Kamdar]

Should we use Option primitive from the https://github.com/yjhmelody/as-container here?

This is Rust-inspired, has decent test coverage, and also allows references that we don't in the Nullable type.