0.12.0

Full Changelog

Rework "has_changed" extension point and replace doctest with Cache2

• BREAKING! rework approach to "calming" made via has_changed customization
  point
  Overloaded free function version of ureact::detail::has_changed() is
  replaced with Customization Point Object
  Existence of operator== is no longer condition for "calming", instead free
  function has_changed is used.
  has_changed is provided only for arithmetic types, enum types and library
  classes ureact::signal and ureact::events.

  Examples of has_changed function:

  1. free function in the same namespace

  namespace foo
  {
      struct Foo
      {
          int value;
      };
      
      constexpr bool has_changed( const Foo lhs, const Foo rhs ) noexcept
      {
          return !( lhs.value == rhs.value );
      }
  }

  2. hidden friend version for the shy ones

  namespace bar
  {
      class Bar
      {
          int value;
         
          friend constexpr bool has_changed( const Bar lhs, const Bar rhs ) noexcept
          {
              return !( lhs.value == rhs.value );
          }
      };
  }

• Remove Codacy badge. Codacy proved to be useless for this repository

• Replace doctest with Cache2. They are mostly the same in terms of interface,
  but activity and support of Cache2 is better

• Move the only benchmark from standalone benchmark folder into ureact_test
  target.
  Cache2 has microbenchmark feature, but I don't understand it's output and
  don't trust it.