[Feature]: Replace `boost::variant` by `boost::variant::variant2`

[jorisv]

🚀 Feature

Replace boost::variant by boost::variant::variant2 to modernize the code base.

This is not mandatory and will break the public API so this feature will be implemented for Pinocchio > 4.

Motivation

Simplify the code base:

• Use lambda instead of Boost::fusion and custom visitors
• Remove BOOST_MPL_LIMIT_LIST_SIZE and BOOST_MPL_LIMIT_VECTOR_SIZE define
• Remove Boost::fusion

Improve build time

crba, rnea, aba and kinematics build benchmark:

• variant
  • 58.79s user 2.71s system 293% cpu 20.936 total
  • 908K src/libpinocchio_default.so.3.8.0
  • Memory: 1992M
• variant2
  • 46.83s user 2.14s system 255% cpu 19.148 total
  • 896K src/libpinocchio_default.so.3.8.0
  • Memory: 1716M

Total build time is reduced, but parallel build time is not because aba.cpp is mainly Eigen3 template instantiation bound.

Alternatives

No response

Additional context

No response

Checklist

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