Try to use From impl declared in JubJub and Bls repos to convert between Scalar types.

[CPerezz]

We have a From impl which we use across all of our repos to go from JubJubScalar -> BlsScalar. But it is not used always in this repo. And we should do it.

This was probably included in #292 & #255

See the lack of usage:

plonk/src/constraint_system/ecc/scalar_mul/fixed_base/mod.rs

Line 35 in a879624

let raw_bls_scalar = composer.variables.get(&jubjub_scalar).unwrap();