The scalar to be multiplied by should be at most `num_limbs` long

[mmagician]

Description

Fixes(?) the failing tests in https://github.com/arkworks-rs/curves/pull/158.

There's a small problem: The current default method for mul_bigint operates on limbs, and so works for any scalar, also those larger than MODULUS. On the other hand, the GLV interface assumes that the scalar is already an element of the ScalarField:

fn glv_mul_projective(p: Projective<Self>, k: Self::ScalarField) -> Projective<Self>;

And so to call the GLV implementation, when we convert from limbs to ScalarField, we assume that the scalar has already been mod reduced: https://github.com/arkworks-rs/curves/blob/a39aa8480e216fb3193139747e149b252ebfc1fd/ed_on_bls12_381_bandersnatch/src/curves/mod.rs#L144 -> which panics when #limbs > N, thus causing the tests to fail.

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[Pratyush]

Maybe we should remove the mul_bigint entirely...