Multiple apps and the event loop

[noahgibbs]

It would be really useful to have SpaceShoes be multi-app, supporting multiple Shoes.app() sections in Ruby scripts. There are a few things that will need to happen:

• Scarpe multi-app support, likely using this PR
• Modify SpaceShoes' event loop code and FEATURES constant to use that multi-app support
• Prefix added DOM elements in wasm_calls.rb with a per-app tag, so that an app can be certain it's only finding/modifying its own elements (the multi_app branch code is a start in this direction, but isn't at all prod-ready)