Improve build times

[aszepieniec]

The biggest contributor to Triton VM's build times is the build script build.rs, of which (presumably) the biggest culprit is automatic degree lowering. This step is currently purely sequential. There are probably ways to parallelize (at least parts of) it, and possibly other ways to speed this step up.

Additionally, the build script generates code. This step might be another candidate for performance optimization.

• Benchmark build.rs step's performances
• Identify ways to improve performance of the slow steps
• Make the to-dos on this list more concrete

It might be beneficial (or even necessary) to overhaul the member crate triton-constraint-circuit to achieve these goals.