Implement Wasm proposal: reference-types

[Robbepop]

This is about adding support for the reference-types Wasm proposal.

The reference-types Wasm proposal has already been included into the Wasm standard and its implementation effort is medium.
Probably takes some days or a few weeks to get it working properly, with tests and documentation.

We should generally keep up with supporting finished Wasm proposals as this allows us to keep the Wasm spec test suite up to date.