This commit adds an example of executing the `wasm2asm` tool to generate asm.js
output instead of WebAssembly. This is often useful when supporting older
browsers, such as IE 11, that doesn't have native support for WebAssembly.
Currently errors are reported via Rust panics but there's lots more errors being
added over time so this commit starts the movement towards the `failure` crate
to more idiomatically report errors as well as provide better error messages
over time.
