Improve JS and WebAssembly support

[booniepepper]

Running dt from WebAssembly and calling the main() function (via _start()) is a bit involved.

(This is my first time ever trying to use WebAssembly APIs btw)

dt = null

errstub = called => (...args) => { throw {called, args}; };
logstub = called => (...args) => console.log({called, args});

args_sizes_get = () => 1;
fd_read = fd => fd == 0 ? "1 1 + pl" : "";
fd_fdstat_get = fd => ({ fd });

fetch("static/dt.wasm")
  .then(r => r.arrayBuffer())
  .then(bytes => WebAssembly.instantiate(bytes, {
    wasi_snapshot_preview1: {
      args_get: errstub("args_get"),
      args_sizes_get,
      fd_close: errstub("fd_close"),
      fd_fdstat_get,
      fd_read,
      fd_readdir: errstub("fd_readdir"),
      fd_seek: errstub("fd_seek"),
      fd_write: errstub("fd_write"),
      path_open: errstub("path_open"),
      proc_exit: errstub("proc_exit"),
      random_get: errstub("random_get"),
  }}))
  .then(module => dt = module.instance.exports)

dt._start()

This can run in a browser console on https://dt.plumbing and will fail (for now on purpose) when trying to call fd_write

The above code could be fleshed out and refactored into something reasonable. It could be TS-ified and etc.

That said, it will be a kinda gross JS inter-op to try to fit everything into stdin/stdout file read and writes... Not fun in a browser context, although fine for something that interprets in an OS context, like Wasmer (probably?).

---

A better interface:

• A one-shot WebAssembly function that interprets code
  • Maybe: export fn interpret(code: []const u8) []const u8 { ... }
• A WebAssembly function that can export a Dt instance
  • Maybe: export fn instance() Dt { ... }
  • ...And then is it straightforward to send JS strings? Does it require an extra layer of abstraction?