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

[martinmoene]

Progress, continuation of this comment in #1:

• [x] Add tests
• [ ] Add benchmarks
  • [ ] nonstd::invoke
  • [ ] nonstd::apply
  • [ ] std::invoke
  • [ ] std::apply
• [x] Add CI:
  • [x] Add GitHub Actions CI configuration
  • [x] Add Appveyor CI configuration
• [x] Add demo pages
  • [x] Create Compiler Explorer page
  • [x] Create Wandbox page
• [ ] Add Packaging,
  • [x] CMake
  • [ ] Create Conan PR
  • [ ] Create vcpkg PR
• [ ] Complete Readme:
  • [x] 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 :)