Absorb full field elements rather than repacked field elements

[winderica]

Previously we generate bit challenges in the following way:

1. Squeeze field elements from the transcript
2. Decompose field elements into bits
3. Repack bits into field elements
4. Absorb repacked field elements

Step 3 is suspicious because the repacked field elements have smaller ranges. Not sure if this has any impact on security, but with this PR, challenge bits are now generated as follows to avoid potential risks:

1. Squeeze field elements from the transcript
2. Absorb full field elements
3. Decompose field elements into bits