types/description: make_description should be constexpr

This would be possible in the general case, but is fine for fundamental
types.

test/introspection.cpp

@@ -1,3 +1,4 @@
+
 #include "introspection/name.hpp"
 #include "introspection/type.hpp"
 #include "std.hpp"

types/description.hpp

@@ -249,6 +249,7 @@ namespace cruft::types {
 
     //-------------------------------------------------------------------------
     template <typename T>
+    constexpr
     description
     make_description (void) noexcept
     {