add is_function_pointer

is_function_pointer like is_member_function_pointer
```
// 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_FUNCTION_POINTER_HPP
#define BOOST_TYPE_TRAITS_IS_FUNCTION_POINTER_HPP

#include
#include
#include

namespace boost
{
template
struct is_function_pointer : public integral_constant
<
bool,
is_pointer::value && is_function::type>::value
> {};

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

#endif // BOOST_TYPE_TRAITS_IS_FUNCTION_POINTER_HPP
```