Use Fx Hash to speed up hash maps in the compiler

[Razican]

We currently use standard library hash maps (std::collections::HashMap) in Boa to organize all of our internal slots and more. This is not very performant, and Fx Hash has just a bit lower DoS resistant, while providing much better performance.

We do not care about DoS in a compiler, usually, so this should improve our performance greatly, especially during the execution.

[Razican]

I did a bit of profiling with callgrind, and more than 10% of our instructions are related to HashMaps, so I'll try to do this optimisation for a quick win :)

I will use rustc-hash, that seems to be the maintained version.