This commit introduces `from_var_bytes` for `Scalar`, a function that
will take arbitrary slices, hash with BLAKE2b into 256-bit numbers, and
perform a modulus multiplication to produce valid scalars.

Such functionality is required for downstream protocols such as Phoenix
or Rusk contract ID computation. They produce arbitrary bytes, being
either asymmetric key exchange protocols or network transaction payload,
annd perform off-circuit field operations with such inputs.