Change printed representation of hex digests

[wwared]

Currently, we're directly printing the 124-ish bit hex representation for BigNums and commitments. We want to change it to print a 128-bit representation, where each 4-byte chunk is aligned at the u32 boundary to BabyBear field elements.

• Rename BigInt to FieldBuffer (or similar)
• Switch the reading/printing to use a flat 8xu32 array, validating that there is no overflow in any field element when reading
• Update hashes in tests and user-manual documentation