Add host module for ww proc API.

[lthibault]

Stacked on top of #109 & #110. Merge those first ✔️

The scope of this PR has expanded a little bit since I started, so I'm cutting it off here. At present, it adds:

1. pkg/csp/proc: a WASM host module that provides a Cap'n Proto RPC transport between the host and guest code.
2. guest/tinygo: a library for writing processes written in Go using TinyGo

Follow-up work:

• Pass a Host capability (or equivalent) to pkg/csp/proc.Module (currently, the guest sees a null capability)
• Define and implement top-level guest API in guest/tinygo
• Unit tests, benchmarks & performance tuning
• Mechanism to limit which capabilities are passed into a process
• Guest libraries for other languages (maybe we should do JS next?)

What is a host module?

A host module is native Go code that the webassembly binary can call. It's webassembly module whose functions are implemented by Wetware itself.

What is guest/tinygo?

guest/tinygo is a client library for users who wish to write Wetware processes in Go, using TinyGo. When imported, it will provide access to cluster capabilities like pubsub, processes, etc. It is not yet fully implemented, but it does succeed in negotiating an rpc.Conn with the host.

Users are able to write processes as ordinary Go programs, e.g.:

package main

import (
    "fmt"


    ww "github.com/wetware/ww/guest/tinygo
)

func main() {
    // get the root anchor; this functionality is WIP.
    it, release := ww.Root().Ls(context.Background())  
    defer release()

    for name := it.Next(); name != ""; name = it.Next() {
         fmt.Println(name)  // This works, though!  Feel free to try hello world!
    }
}

To compile: tinygo -o main.wasm -scheduler=asyncify -target=wasi main.go
To run: ww run main.wasi

[mikelsr]

Looks good, thank you for taking the time to do it and talk it with me!

[lthibault]

Converting to draft. I'm going to try to break this into a few smaller PRs.

[lthibault]

I think this has been superseded. Closing, but happy to reopen if needed.