Sunsetting Pinocchio 2 support (officially)

[ManifoldFR]

Aligator 0.13 will be the last version which (we pretend) supports Pinocchio 2.

The binaries distributed on conda-forge are already only compiled against the latest Pinocchio (through the conda-forge global pinning if I understand what @jorisv explained to me correctly).

After this, all include guards, macros in the headers to protect against using Pinocchio 3 features or includes will be removed, and users will officially not be able to build aligator against Pinocchio 2. Some bugs in the current codebase might prevent Pinocchio 2 support in practice, but now it will be official.