Verify that all downcasts are represented by a `Cast` node

[Derppening]

Currently, the Wasm backend emits some downcasts of its own rather than fully relying on the presence of a Cast node. This causes:

• The IR to miss crucial information on when cast should be inserted
• The Wasm backend's downcast implementation to do more work than is necessary

Preferably, we would like to have Wasm's downcast helper to only cast between nullable and non-null types, since these kinds of casts cannot be represented in the IR.

A WIP implementation is available here: https://github.com/Derppening/mlscript/tree/enhance/more-casts-in-typed-ir. One major issue with the implementation is the large number of explicit Cast that are present in the diff - I suspect enabling typed-annotated let bindings (#547) may address some of this.

[LPTK]

The solution to the proliferation of casts will be the new resolution pass, which should also deal with the let case without making programmers add explicit annotations.