Separate Circuit Construction from Witness Assignment

[bhgomes]

In arkworks and most other libraries for building circuits, the same "circuit generation pipeline" is used where at the definition of the circuit we use a callback that generates the circuit description for use during "compile-time", when defining the proving/verifying keys, and during "proving-time", when the proof is constructed. For efficiency and canonicity of the circuit we should separate the circuit definition phase from the witness assignment phase.