This package update provides two new libraries
- Contract (compiled library) [1]: Contract programming for C++.
  All contract programming features are supported: Subcontracting,
  class invariants, postconditions (with old and return values),
  preconditions, customizable actions on assertion failure (e.g.,
  terminate or throw), optional compilation and checking of
  assertions, etc, from Lorenzo Caminiti.

- HOF (header-only library) [2]:
  Higher-order functions for C++, from Paul Fultz II.

More info can be found at the usual place [3].

[1]: https://www.boost.org/libs/contract
[2]: https://www.boost.org/libs/hof
[3]: https://www.boost.org/users/history/version_1_67_0.html

Signed-off-by: Carlos Miguel Ferreira <carlosmf.pt@gmail.com>