Use hybrid C / Pascal strings in the evaluator

Replace the null-terminated C-style strings in Value with hybrid C /
Pascal strings, where the length is stored in the allocation before the
data, and there is still a null byte at the end for the sake of C
interopt.

Co-Authored-By: Taeer Bar-Yam <taeer@bar-yam.me>
Co-Authored-By: Sergei Zimmerman <sergei@zimmerman.foo>

src/libexpr/eval-cache.cc

@@ -147,7 +147,7 @@ struct AttrDb
                 for (auto * elem : *context) {
                     if (!first)
                         ctx.push_back(' ');
-                    ctx.append(elem);
+                    ctx.append(elem->view());
                     first = false;
                 }
                 state->insertAttributeWithContext.use()(key.first)(symbols[key.second])(AttrType::String) (s) (ctx)