add is_bool

is_bool.hpp
```
// Distributed under the Boost Software License Version 1.0 https://www.boost.org/LICENSE_1_0.txt
// Copyright Gero Peterhoff

#ifndef BOOST_TYPE_TRAITS_IS_BOOL_HPP
#define BOOST_TYPE_TRAITS_IS_BOOL_HPP

#include
#include

namespace boost
{
template struct is_bool : public integral_constant
<
bool,
is_same::type, bool>::value
> {};

#if !defined(BOOST_NO_CXX17_INLINE_VARIABLES)
template inline constexpr bool is_bool_v = is_bool::value;
#endif
} // boost

#endif // BOOST_TYPE_TRAITS_IS_BOOL_HPP
```

Start by reviewing the proposed is_bool.hpp content in the issue and nearby Boost.TypeTraits headers for naming and layout conventions. The work is done when the requested is_bool trait and is_bool_v definition are added in that header.

Written by the indexing model from the issue text.