Portable WASM extensions

WASM has a few properties which make it well-suited for use in Jsonnet:
* It offers portability through having well-defined semantics.
* If doesn't assume anything (in the core version) about the environment in which it runs.
* It has a good and precise spec.

It should be possible to create a standard of portable pure extensions for Jsonnet using WASM. This may alleviate what is probably the biggest shortcoming of Jsonnet – that it is practically impossible to have high-performance libraries for things like parsing new formats or markdown processing. And using WASM for that wouldn't require any compromise on Jsonnet ideals.

Also, I think that in general the idea of efficient and portable pure functions has a big potential and this is a practical and useful way to explore it.