* begin adding verification checks

* add verifier checks

* remove unnecessary dead_code

* remove unneeded tracking and checks

* remove unused error type

* Separate types for Relaxed R1CS and R1CS instances and witnesses
* Allows creating default values for Relaxed R1CS types
* StepSNARK now folds a regular R1CS instance-witness into a running Relaxed R1CS instance-witness
* We additionally enforce input chaining checks: the incoming instance must have input that matches the output of the incremental computation thus far