host-gen-js: Output optimization

[guybedford]

This is based to #415, which is in turn based to #414, so should land after those.

Updates the output to be more friendly to minifiers resulting in a smaller total JS file size - arrow functions are used for intrinsics, and the dataView(memory) pattern is replaced with a dedicated dataView{n} binding per memory, which should be able to be optimized out if not used for a particular memory.

[alexcrichton]

This looks reasonable, but the dataView optimization I don't think will work because memory.grow can invalidate the prior ArrayBuffer so JS needs to always check to see if the ArrayBuffers match to get the most up-to-date view of memory.

[guybedford]

Ah, ok I can't come up with a better alternative, will close this then.