Initial work for the Fibonacci demo prover

[weikengchen]

Since we need to replace the channel, it seems to force us to copy-paste some code from stwo and make modifications over it.

In this PR, we will put those code under crate::stwo_fork, and we will revisit in a later day what changes should we suggest to stwo to avoid the need of forking (mostly due to hardcoded struct).

Some of the immediate next steps are labeled in the code as "TODO(Bitcoin)".

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 95.38462% with 3 lines in your changes are missing coverage. Please review.

Files	Coverage Δ
src/channel/bitcoin_script.rs	95.23% <100.00%> (+7.73%)	:arrow_up:
src/circle/bitcoin_script.rs	100.00% <100.00%> (+18.18%)	:arrow_up:
src/constraints/bitcoin_script.rs	100.00% <ø> (ø)
src/fri/bitcoin_script.rs	100.00% <100.00%> (+4.76%)	:arrow_up:
src/fri/mod.rs	100.00% <ø> (ø)
src/lib.rs	50.00% <ø> (ø)
src/merkle_tree/bitcoin_script.rs	100.00% <ø> (ø)
src/merkle_tree/mod.rs	82.85% <ø> (+0.50%)	:arrow_up:
src/oods/bitcoin_script.rs	100.00% <100.00%> (ø)
src/oods/mod.rs	100.00% <100.00%> (ø)
... and 3 more

[weikengchen]

Relocating the stwo_fork to a separate repo, for two reasons: (1) stwo's license is Apache2 while this repo's license is MIT, so it is not very suitable to mix them together (2) coverage