🔨 Remove static from internal function

During refactoring of `not_null`, the `detail::not_null_to_address`
was relocated from a `static` member function of `not_null` to being
a `detail` function -- though it unintentionally retained the `static`
identifier.

This appears to potentially be confusing MSVC, and is objectively wrong
already since this forces the symbols to have internal linkage. These
modifiers have been removed.

include/not_null.hpp

@@ -140,10 +140,10 @@ inline namespace bitwizeshift {
     /// \param p the pointer-like type
     /// \return the underlying raw pointer
     template <typename U>
-    static constexpr auto not_null_to_address(U* p)
+    constexpr auto not_null_to_address(U* p)
       noexcept -> U*;
     template <typename Ptr>
-    static constexpr auto not_null_to_address(const Ptr& p)
+    constexpr auto not_null_to_address(const Ptr& p)
       noexcept -> decltype(::NOT_NULL_NS_IMPL::detail::not_null_to_address(p.operator->()));
     /// \}