Initial refactoring of the channel

[weikengchen]

This PR mainly wants to use a Sha256Channel that implements the Channel trait and can replace the Blake2 channel used in stwo for proof generation. This has been done in the stwo fork.

A few changes are made:

• update the README on the terminology about absorbing
• remove Commitment and CommitmentGadget and move the corresponding functions to utils
• use stwo fork's implementation for Sha256 channel
• update the fibonacci ref implementation to use Sha256 channel

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 94.73684% with 3 lines in your changes are missing coverage. Please review.

Files	Coverage Δ
src/fri/bitcoin_script.rs	100.00% <100.00%> (ø)
src/fri/mod.rs	100.00% <100.00%> (ø)
src/lib.rs	75.00% <100.00%> (+25.00%)	:arrow_up:
src/merkle_tree/bitcoin_script.rs	100.00% <ø> (ø)
src/merkle_tree/mod.rs	84.37% <100.00%> (+2.02%)	:arrow_up:
src/oods/bitcoin_script.rs	100.00% <ø> (ø)
src/oods/mod.rs	100.00% <100.00%> (ø)
src/utils/bitcoin_script.rs	100.00% <100.00%> (ø)
src/utils/mod.rs	96.22% <100.00%> (+6.22%)	:arrow_up:
src/channel/bitcoin_script.rs	94.73% <50.00%> (-0.72%)	:arrow_down:
... and 1 more