Fix build error when "yoloproofs" enabled

[matthiasgeihs]

Currently, the crate doesn't build with "yoloproofs" enabled.

error[E0599]: no method named `ok_or` found for struct `CtOption` in the current scope
   --> src/r1cs/proof.rs:180:59
    |
180 |         let t_x = Scalar::from_canonical_bytes(read32!()).ok_or(R1CSError::FormatError)?;
    |                                                           ^^^^^ method not found in `CtOption<Scalar>`

Since curve25519-dalek v4, from_canonical_bytes returns CtOption.
Solution: Wrap with Option::from.

[cathieyun]

LGTM, thanks for the fix.