Fix deserialization for modulo with 64 shaving bits

[weikengchen]

Description

In an application, I need to instantiate a field in which the modulus is 256 bits. In arkworks-rs, this field must use Fp320 instead of Fp256, and therefore the number of shaving bits is 64 bits.

However, this can cause an issue during deserialization.

This PR fixes this issue.

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N/A:

• Wrote unit tests
• Updated relevant documentation in the code