Feature: Support overloads in dispatch definition

[mingxwa]

Overloading is a widely used paradigm that helps reducing the effort of naming of abstraction. However, it is not supported in the existing implementation of proxy. This PR aims to add this capability inspired by #43. After this change, overloading with different signatures becomes available in a single dispatch. An operator() was also added to proxy to simplify the invocation syntax when only one dispatch is specified. Unit test cases are updated accordingly to cover this change. Documentations will be updated later.

BREAKING CHANGE: pro::dispatch no longer supports adaption with CPO. This feature has been moved to another class template pro::dispatch_adaptor. pro::dispatch<std::size_t(), std::ranges::size> was a valid type before this change but not anymore. The equivalent type has become pro::dispatch_adaptor<std::ranges::size, std::size_t()> with this change.

The syntax to define a dispatch with single overload is the same as before:

struct foo : pro::dispatch<int(int)> {
  auto operator()(const auto& self, int val) { return self.foo(val); }
};

The syntax to define a dispatch with multiple overloads is similar:

struct foo : pro::dispatch<int(int), double(double)> {
  auto operator()(const auto& self, auto val) { return self.foo(val); }
};

See the updated unit test cases for more details.