fix(ssa refactor): Speedup acir-gen

[jfecher]

Description

Problem*

Resolves #1750

Summary*

After a flamegraph profiling I found out that most time in acir-gen for the eddsa test was spent in mul_var and sub_var, and most time in those methods was spent in TwoWayMap::insert where to insert a new AcirVar, AcirVarData pair we were calling FieldElement::eq repeatedly which uses to_bytes_be internally.

To fix this, I changed the TwoWayMap to a single-way map. The downside of this fix is if an identical AcirVarData is ever tried to be inserted, it will be assigned a new id instead of an existing one. This may slow down proving later on.

Documentation

• This PR requires documentation updates when merged.

  • I will submit a noir-lang/docs PR.
  • I will request for and support Dev Rel's help in documenting this PR.

Additional Context

PR Checklist*

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.

[jfecher]

It is worth noting that nargo prove --experimental-ssa still takes 62s on my machine for eddsa in release mode. Most of the time is spent on proving. Only the first second is spent on the rest of the compiler.