feat: add example for printing constraints

[hawkadrian]

Adds example showing how to print constraints from compiled constraint system using type assertion and GetR1Cs()/GetSparseR1Cs() methods.

Fixes #1640

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Note

Adds a new example package showing how to compile a circuit and print R1CS/SparseR1CS constraints and stats on BN254.

• Examples:
  • examples/print_constraints:
    • Adds CubicCircuit (x^3 + x + 5 == y).
    • New examples to compile on BN254 with r1cs.NewBuilder and scs.NewBuilder, type-assert to cs_bn254.R1CS/SparseR1CS, fetch constraints via GetR1Cs/GetSparseR1Cs, and print system stats and each constraint using String().

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