Create WIT definitions for lunatic's host functions

[bkolobara]

We should provide *.wit files for lunatic's host functions. WIT (WebAssembly Interface Types) files are definitions of higher level types and function signatures that describe an API that the Wasm host is exporting and the guest is importing. It can also be an interface between two Wasm modules in the component model.

For example, the WASI functions that are provided by lunatic have a formal spec in the *.wit file format, but none of the other ones (networking, process spawning, messages, ...) do. I think we should also create them! However, this will result in some breaking API changes.

There are two main benefits in providing such a spec:

• It provides documentation.
• It can be used to automatically generate guest bindings for many languages or host bindings inside of lunatic.

Even if we don't end up using the automatically generated bindings, following the canonical ABI proposal for host functions will result in wider support from guests. Lunatic is already exposing an unspecified inconsistent ABI that is fairly similar to the proposal. So, why not go a step further and just use the standard everyone is using?

This will also give us an opportunity to go over all the existing host functions and remove inconsistencies.