Remove ::template method ugliness when using templated Methods

[Anton3]

See a snippet from examples:

template<typename Signature>
using function_ref = aa::poly_ref<aa::call<Signature>::template method>;

This is required because AnyAny works with template template parameters.

It wouldn't be required if we slightly modified the way Methods are defined.

Before:

template <typename T>
struct Foo {
  static void do_invoke(T&) { ... }

  template <typename CRTP>
  struct plugin {
    void foo() const { ... }
  };
};

After:

struct Foo {
  template <typename T>
  static void do_invoke(T&) { ... }

  template <typename CRTP>
  struct plugin {
    void foo() const { ... }
  };
};

It has the additional benefit that it forces plugin not to depend on T, purely syntactically.

[kelbon]

Method now is a typename and not template typename