Add WebAssembly target

[mverleg]

Try to have both native and WebAssembly. If that complicates things too much or takes a lot of time, then only WebAssembly is also an option (for now).

[mverleg]

https://www.hellorust.com/setup/wasm-target/
https://www.hellorust.com/demos/add/index.html
http://thecodebarbarian.com/getting-started-with-webassembly-in-node.js.html

How to run wasm locally so I can do tests?

How to wrap it in JS? I need at least loading and delegate to a webworker.

[mverleg]

There's a shell here: https://github.com/WebAssembly/binaryen

And a book here: https://rust-lang-nursery.github.io/rust-wasm/setup.html

[mverleg]

My current plan is to only target wasm for now, using binaryen IR as target to get optimizations. Skip LLVM for now, web is the primary (first) target and won't need heavy optimization as that likely happens in the browser anyway. Binaryen can do some optimizations on wasm for optimized mode (might break source map).

[mverleg]

This got a little mixed up; split into:

#47 for the compiler itself
#48 for a target to compile Mango to