Add ww run command.

[lthibault]

This PR provides a command for executing WASM binaries locally. It is part of the workstream to break #111 into smaller chunks.

Commented code will be uncommented in a follow-up commit that adds piecemeal support for the ww host module.

If you want to test this, try compiling the following with tinygo -o main.wasm -target=wasi main.go, and running it with ww run main.wasm.

package main

import "fmt"

// main.go

func main() {
    fmt.Println("hello, wetware!")
}