libexpr: Factor out Payload union to a default implementation of ValueStorage

This factors out most of the value representation into a mixin class.
`finishValue` is now gone for good and replaced with a simple template
function `setStorage` which derives the type information/disriminator from
the type of the argument. Likewise, reading of the value goes through function
template `getStorage`.

An empty type `Null` is introduced to make the bijection InternalType <-> C++ type
complete.

src/libexpr/primops.cc

@@ -5050,7 +5050,7 @@ void EvalState::createBaseEnv(const EvalSettings & evalSettings)
 
     /* Now that we've added all primops, sort the `builtins' set,
        because attribute lookups expect it to be sorted. */
-    getBuiltins().payload.attrs->sort();
+    const_cast<Bindings *>(getBuiltins().attrs())->sort();
 
     staticBaseEnv->sort();