Port RtoAC to WebAssembly for performance

Closes #82 (closed).

The following benchmark now runs slightly faster than native Clean (5.7s vs 6s) on x64 linux because native Clean uses sprintf:

import StdEnv
Start = removeDup [toString 123.456 \\ _ <- [0..10000000]]

The C interpreter takes 8s. The Wasm version is about 3.3x faster now as the old version took 18.8s.