New skill Dereferencable, adding constexpr, adding [[nodiscard]], and other fixes

[pemessier]

Summary of the proposed changes:

• Make ostream operator<< enabled only if the skill Printable is present
  • operator<< is currently always defined for all types. This may have side effects in some cases, like using Boost.Test. It should be generated only when using the Printable skill.
  • It is now enabled only when the Printable skill is set.
  • Added a unit test.
• Add skill Dereferencable (#52)
• Added tests for PreIncrementable, PostIncrementable, PostIncrementable
  • Also fixed compilation for at least MSVC for PostIncrementable and PostDecrementable skills
• Added constexpr for all skills
• Some operators are only constexpr under C++17 and over
• Adding support for [[nodiscard]] if available

Travis CI is ok, and works well on MSVC

[joboccara]

Thanks for the PR!