Fast compilation mode

[vouillon]

It might make sense to do without Binaryen when debugging since it can be slow. For that, we need to generate directly some Wasm binary code, with source maps (#93).

With separate compilation, we use wasm-merge only to compile and link the runtime files (from the different linked libraries). It is probably ok to keep this since that should be fast.

We probably want to implement some optimizations to still generate decent code:

• reduce the number of local variables by taking advantage of the Wasm stack and by coalescing them
• eliminate clearly unnecessary casts and boxing operations.