`wasm` compilation significantly slower compared to `binary`

[bee344]

When compiling contracts that have many dependencies, or a large batch of contracts, the compilation times increase significantly when compared to using the resolc binary to compile. In my case, compiling 514 contracts takes ~41 secs with the binary vs ~8 mins with the wasm.

cc: @smiasojed

[xermicus]

There are two obvious reasons:

• The parallel feature, controlling whether Projects are processed in parallel, is disabled when compiling to emscripten.
• Additionally, executing Wasm is expected to be slower than native code.

The reason for disabling parallelization was resource pressure in the browser, however for non-browser environments it might not be a problem. Disabling dynamically in browser environments (instead of disabling it at compile time) should help a lot. Additionally there are some performance improvements planned too.