Document `r1cs-core`, `r1cs-std`, and `crypto-primitives`

[Pratyush]

• [ ] Add basic documentation to all items.
  • [ ] r1cs-core
  • [x] r1cs-std
  • [ ] crypto-primitives
• [ ] Add doc-tests for important items.
  • [ ] r1cs-core
  • [x] r1cs-std
  • [ ] crypto-primitives
• [ ] Add integration examples in important modules.
  • [ ] r1cs-core
  • [x] r1cs-std
  • [ ] crypto-primitives