feat(solc): add support for compiling solc in parallel

[mattsse]

Motivation

If the svm feature is used, contracts are grouped by their required solidity version and compiled separately. Instead of processing this sequentially, we can spawn multiple processes concurrently using the async_compile functions which is faster.

Solution

• Add Solc::compile_many -> CompileMany which effectively works like a FuturesUnordered capped at a certain number, (num_cpus in our case).
• add a job parameter in Project which sets this threshold, (default, num_cpu)
• when calling svm_compile we check if we can use compile_many and use that instead

I was able to unify some stuff and refactor some things, but there is probably still some code duplication in those to branches (jobs == 1 || jobs > 1) when it comes to artifact and cache handling.

needs some further testing with forge.

PR Checklist

• Added Tests
• Added Documentation
• Updated the changelog

[gakonst]

lgtm - pending further performance improvements on a per-file level