Bring invoke lite to the level of other 'lite' projects

[martinmoene]

Progress, continuation of this comment in #1:

• Add tests
• Add benchmarks
  • nonstd::invoke
  • nonstd::apply
  • std::invoke
  • std::apply
• Add CI:
  • Add GitHub Actions CI configuration
  • Add Appveyor CI configuration
• Add demo pages
  • Create Compiler Explorer page
  • Create Wandbox page
• Add Packaging,
  • CMake
  • Create Conan PR
  • Create vcpkg PR
• Complete Readme:
  • Add example
  • Complete in a nutshell
  • Provide implementation status in table
  • Other implementations of invoke
  • Notes and references
  • Add Test specification
• TBD

[pfeatherstone]

I'm surprised std::tuple_size works with std::array and std::pair

[pfeatherstone]

Yep, apparently it's in the standard. gosh

[martinmoene]

Yeah, discovered it while thinking i would have to do work myself. It's also not entirely unexpected :)