ASM.js support

[gfwilliams]

Originally: https://github.com/espruino/EspruinoTools/issues/9 And also: https://github.com/espruino/Espruino/issues/332

It'd be nice to make sure we compile asm.js code relatively efficiently. I think the main requirements are:

• Infer argument types: https://github.com/gfwilliams/EspruinoCompiler/issues/4
• Make sure we access arraybuffers such as MEM8 in a fast way (ideally getting a pointer and dealing directly with that)