Constexpr next (#789)

Implements constexpr: nextafter, nextafterf, nextafterl, nexttoward, nexttowardf, and nexttowardl as described in P0533R9

doc/sf/ccmath.qbk

@@ -9,7 +9,7 @@ LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
 
 [heading Description]
 
-`Constexpr` implementations of the functionality found in `<cmath>`.
+`Constexpr` implementations of the functionality found in `<cmath>` and `<cstdlib>` [@https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/p0533r9.pdf proposed for C++23].
 In a `constexpr` context the functions will use an implementation defined in boost.
 If the context is not `constexpr` the functionality will be directly from the STL implementation of `<cmath>` used by the compiler.
 All functions that take an `Integer` type and return a `double` simply cast the `Integer` argument to a `double`.
@@ -187,7 +187,13 @@ All of the following functions require C++17 or greater.
         Requires compiling with fma flag
 
         template <typename Arithmetic1, typename Arithmetic2, typename Arithmetic3>
-        inline constepxr Promoted fma(Arithmetic1 x, Arithmetic2 y, Arithmetic3 z) noexcept
+        inline constexpr Promoted fma(Arithmetic1 x, Arithmetic2 y, Arithmetic3 z) noexcept
+
+        template <typename Arithmetic1, typename Arithmetic2>
+        constexpr Promoted nextafter(Arithmetic1 from, Arithmetic2 to)
+
+        template <typename T>
+        constexpr Promoted nexttoward(T from, long double to)
 
     } // Namespaces