Remove std::string alias (for real this time)

Also use std::string_view in a few more places.

src/libstore/dummy-store.cc

@@ -21,7 +21,7 @@ struct DummyStore : public virtual DummyStoreConfig, public virtual Store
         , Store(params)
     { }
 
-    string getUri() override
+    std::string getUri() override
     {
         return *uriSchemes().begin();
     }
@@ -43,8 +43,11 @@ struct DummyStore : public virtual DummyStoreConfig, public virtual Store
         RepairFlag repair, CheckSigsFlag checkSigs) override
     { unsupported("addToStore"); }
 
-    StorePath addTextToStore(const string & name, const string & s,
-        const StorePathSet & references, RepairFlag repair) override
+    StorePath addTextToStore(
+        std::string_view name,
+        std::string_view s,
+        const StorePathSet & references,
+        RepairFlag repair) override
     { unsupported("addTextToStore"); }
 
     void narFromPath(const StorePath & path, Sink & sink) override