CppCon 2019 design

• The design as described on CppCon 2019 talk (https://youtu.be/0YW6yxkdhlU)
• Applied the feedback from the Cologne evening session
  • upcasting_traits renamed to downcasting_traits
  • Dimension template parameter removed from quantity
• units moved to a std::experimental namespace
• Leading underscore prefix removed from UDLs
• Added a few more derived dimensions
• meter renamed to metre
• Missing operator* added
• Predefined dimensions moved to a dedicated directory
• dimension_ prefix removed from names of derived dimensions
• cmcstl2 library updated to 2019.09.19
• base_dimension is a value provided as const& to the exp type
• integrated with Compiler Explorer
• gsl-lite dependency removed
• Fractional dimension exponents support added
• QuantityOf concept introduced
• quantity_cast<U, Rep>() support added