Change String_Schema to only hold PreValue objects

The API does not guarantee yet that everything stored in
`String_Schema` is a Value, although it doesn't make much
sense to allow anything else and as it turns out we don't!
Use steamroller tactics to ensure we don't store anything
else after evaluation from Binary_Expression.

src/ast.hpp

@@ -1703,16 +1703,16 @@ namespace Sass {
   // Interpolated strings. Meant to be reduced to flat strings during the
   // evaluation phase.
   ///////////////////////////////////////////////////////////////////////
-  class String_Schema : public String, public Vectorized<Expression_Obj> {
+  class String_Schema : public String, public Vectorized<PreValue_Obj> {
     ADD_PROPERTY(bool, css)
     size_t hash_;
   public:
     String_Schema(ParserState pstate, size_t size = 0, bool css = true)
-    : String(pstate), Vectorized<Expression_Obj>(size), css_(css), hash_(0)
+    : String(pstate), Vectorized<PreValue_Obj>(size), css_(css), hash_(0)
     { concrete_type(STRING); }
     String_Schema(const String_Schema* ptr)
     : String(ptr),
-      Vectorized<Expression_Obj>(*ptr),
+      Vectorized<PreValue_Obj>(*ptr),
       css_(ptr->css_),
       hash_(ptr->hash_)
     { concrete_type(STRING); }

src/eval.cpp

@@ -663,9 +663,9 @@ namespace Sass {
                                                     b->op(), s_l->last(), b->right());
         bin_ex->is_delayed(b->left()->is_delayed() || b->right()->is_delayed()); // unverified
         for (size_t i = 0; i < s_l->length() - 1; ++i) {
-          ret_schema->append(s_l->at(i)->perform(this));
+          ret_schema->append(Cast<PreValue>(s_l->at(i)->perform(this)));
         }
-        ret_schema->append(bin_ex->perform(this));
+        ret_schema->append(Cast<PreValue>(bin_ex->perform(this)));
         return ret_schema->perform(this);
       }
     }
@@ -676,9 +676,9 @@ namespace Sass {
         Binary_Expression_Obj bin_ex = SASS_MEMORY_NEW(Binary_Expression, b->pstate(),
                                                     b->op(), b->left(), s_r->first());
         bin_ex->is_delayed(b->left()->is_delayed() || b->right()->is_delayed()); // verified
-        ret_schema->append(bin_ex->perform(this));
+        ret_schema->append(Cast<PreValue>(bin_ex->perform(this)));
         for (size_t i = 1; i < s_r->length(); ++i) {
-          ret_schema->append(s_r->at(i)->perform(this));
+          ret_schema->append(Cast<PreValue>(s_r->at(i)->perform(this)));
         }
         return ret_schema->perform(this);
       }