Compile to wasm instead of js

[Nimaoth]

Currently the browser version is based on js

Problems:

• Slow (bad Nim->JS codegen, interaction with DOM (technically not necessary))
• Optimizations are either good for native and bad for js or the other way round
  • E.g. Native prefers object, stack allocation etc., JS prefers ref object and preventing copies
• Lots of wrapping around APIs because JS backend and native backend are so different
• Issues with big int

Advantages of wasm:

• Closer to native backend -> ptr, cast etc work properly
• Better performance

Rendering should probably switch to WebGL

Open questions:

• How to do interface between WASM <-> JS with async functions