Add fast wittness generation for r1cs

[snjax]

Currently, fawkes-crypto decide, should it create new signals or not, by calling as_const method of Signal trait (see example here ) and build both witness and constraint system ([1], [2]).

Signals have complex structures (CNum definition), so, operations with them are not fast.

Also, the constraint system is circuit specific (it does not change at multiple calls), and witness depends on the values of internal signals.

For better performance, we need to split constraint system building and witness building. And prover should work with preloaded from the file constraint system and as_const calls cache.

The as_const calls cache could be stored inside the bit-vector. Also, this data and an additional flag for using the pre-computed constant system could be stored here.

In the case of using precomputed CS, all heavy constraint system-related things when we work on CNum, like LC operations, enforce and enforce_pub should be omitted.

In addition, it is better to store r1cs data in snarkjs-compatible format (look at zkutil for example).