Add support for WASM plugins

[dmbaturin]

Lua-ML integration was a huge step forward for soupault — support for arbitrary element tree manipulation logic enabled a whole bunch of completely new use cases and possibilities. Plugin support is an undeniable great thing for an SSG.

However, even when Lua-ML is made multicore-safe and supports modern Lua features, there's still a problem. Some people just don't like Lua — like me, for instance. ;)
I don't like a lot of Lua's design ideas and I would certainly use another languages for writing plugins, if I had an opportunity.

One possible option is to add support for WASM.

• Pretty much every language can be compiled to WASM these days.
• WASM seems like a reasonably sane (although I'd prefer a register machine).
• The reference implementation of WASM is written in OCaml (although it's not clear how embeddable it is).

We need to find our what it takes to make the reference implementation embeddable in the same way as Lua-ML and how to go from there.

CC: @SoniEx2