Unchecked `WitnessPoint` to `JubJubAffine` conversions

[ureeves]

There are still unchecked conversions in the code. Specifically here. We should what pros and cons of keeping/removing them. If we decide to keep them we should comment them properly.

This was first mentioned in this PR.

[moCello]

File: src/constraint_system/ecc/curve_addition/variable_base_gate.rs

  29   │         let p1 = JubJubAffine::from_raw_unchecked(
  30   │             self.witnesses[&x_1],
  31   │             self.witnesses[&y_1],
  32   │         );
  33   │
  34   │         let p2 = JubJubAffine::from_raw_unchecked(
  35   │             self.witnesses[&x_2],
  36   │             self.witnesses[&y_2],
  37   │         );
 

[ureeves]

The only conversions like these that remain in the code are in composer.rs and pertain to a point->point conversion. This means there are no remaining unchecked scalar->point conversions anymore. Closing.