Septidon compatibility

[naure]

Follow-up to #14 based on @noel2004’s suggestions.

Usage

The implementation of the new shorter permutation is now selected by a feature short like so in Cargo.toml:

poseidon-circuit = { git = "https://github.com/scroll-tech/poseidon-circuit.git", features = ["short"] }

Changes:

• Backward compatibility of the types PoseidonHashConfig and PoseidonHashChip.
• Renamed the generic types to SpongeConfig and SpongeChip.
• Moved the septidon module as a sibling of the old implementation.
• Minimized item visibilities.
• Removed unused constants attributes.
• Moved the spec into the spec folder.

[noel2004]

Great. The code is backward compatible now