refactor: use string accessors

Create context, string_view, and c_str, accessors throughout in order to
better support improvements to the underlying string representation.

src/libexpr/primops/fetchClosure.cc

@@ -133,7 +133,7 @@ static void prim_fetchClosure(EvalState & state, const PosIdx pos, Value * * arg
 
         else if (attrName == "toPath") {
             state.forceValue(*attr.value, attr.pos);
-            bool isEmptyString = attr.value->type() == nString && attr.value->string.s == std::string("");
+            bool isEmptyString = attr.value->type() == nString && attr.value->string_view() == "";
             if (isEmptyString) {
                 toPath = StorePathOrGap {};
             }