We notice that we have 2 available 'tags' in C++ standard library now:

=====
class my_iterator { using iterator_concept =
std::random_access_iterator_tag };
class my_sender { using sender_concept = std::execution::sender_t; }
=====

Oops, the first ends with `_tag` and the second ends with `_t`. But they
share the same effect (to make a class satisfy a particular concept).
Should we unify them?
Hmmm, before that let's see where we use `_tag` and `_t` in C++ standard
library.

xxx_t:
1. <type_traits>. When we have `add_const<T>::type`, we typedef an
`add_const_t<T>`.
2. Typedef which unifies different platforms. `uint8_t`, `wchar_t`,
`clock_t`, etc.
3. Utility type of global constexpr variable. e.g. nullopt_t, unexpected_t,
etc.
4. Utility type of function overload parameters. e.g.
unique_lock(defer_lock_t), vector(from_range_t), etc.

xxx_tag:
1. Customize xxx_concept in class. e.g. using iterator_concept
forward/bidirectional/.../random_access_iterator_tag.

Should we rename `std::execution::sender_t` into
`std::execution::sender_tag`?
Thank you!