Fixed base scalar mul needs to use two limbs to encode the scalar field of grumpkin

[kevaundray]

Aim

The fixed base scalar multiplication algorithm needs more than a single field element to encode the scalar field of the embedded curve.

Expected Behavior

We should change it to have two limbs which works for the curves we currently care about and in the future, we should generalize it with perhaps a byte array abstraction, or if we find out that two limbs is always enough we can stick with two limbs

Bug

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Environment

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Additional Context

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