Bring ConstraintSynthetizer to testudo

[nikkolasg]

Currently, the way to create a R1CS matrix, describing the computation, is manually done here. This is because it's useful for quickly testing, and it come from the Spartan codebase.

• To get better, more uniform benchmark accross proofs system, it's best to compare against similar circuits.
• To actually put in production Testudo, we eventually need this feature of course !

The goal is two fold:

• Bring the ability to write circuits using Arkworks circuit synthetizer from here and example usage here
• Bring the ability to write circuits using Bellman circuit synthetizer from here with example implementation usage here

Bringing arkworks first is priority as it gives us the opportunity to implement a verifier of Testudo itself fairly easily.

• Arkworks constraint synthetizer trait implemented in Testudo
  • Moved tests to use this new functionality
• Bellman constraint synthetizer trait implemented in Testudo
  • Use it as a feature