Crudely make addToStoreFromDump take Source not string

I just as little beyond the type as possible, so the implementation
changes this enables can be reviewed separately.

src/libstore/build.cc

@@ -2774,7 +2774,7 @@ struct RestrictedStore : public LocalFSStore
         goal.addDependency(info.path);
     }
 
-    StorePath addToStoreFromDump(const string & dump, const string & name,
+    StorePath addToStoreFromDump(Source & dump, const string & name,
         FileIngestionMethod method = FileIngestionMethod::Recursive, HashType hashAlgo = htSHA256, RepairFlag repair = NoRepair) override
     {
         auto path = next->addToStoreFromDump(dump, name, method, hashAlgo, repair);

src/libstore/daemon.cc

@@ -410,8 +410,11 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
         logger->startWork();
         if (!savedRegular.regular) throw Error("regular file expected");
 
+        StringSource dumpSource {
+            method == FileIngestionMethod::Recursive ? *savedNAR.s : savedRegular.s
+        };
         auto path = store->addToStoreFromDump(
-            method == FileIngestionMethod::Recursive ? *savedNAR.s : savedRegular.s,
+            dumpSource,
             baseName,
             method,
             hashAlgo);

src/libstore/local-store.cc

@@ -1033,9 +1033,12 @@ void LocalStore::addToStore(const ValidPathInfo & info, Source & source,
 }
 
 
-StorePath LocalStore::addToStoreFromDump(const string & dump, const string & name,
+StorePath LocalStore::addToStoreFromDump(Source & dumpSource, const string & name,
     FileIngestionMethod method, HashType hashAlgo, RepairFlag repair)
 {
+    // FIXME: See if we can use the original source to reduce memory usage.
+    auto dump = dumpSource.drain();
+
     Hash h = hashString(hashAlgo, dump);
 
     auto dstPath = makeFixedOutputPath(method, h, name);

src/libstore/local-store.hh

@@ -153,7 +153,7 @@ public:
        in `dump', which is either a NAR serialisation (if recursive ==
        true) or simply the contents of a regular file (if recursive ==
        false). */
-    StorePath addToStoreFromDump(const string & dump, const string & name,
+    StorePath addToStoreFromDump(Source & dump, const string & name,
         FileIngestionMethod method = FileIngestionMethod::Recursive, HashType hashAlgo = htSHA256, RepairFlag repair = NoRepair) override;
 
     StorePath addTextToStore(const string & name, const string & s,

src/libstore/store-api.hh

@@ -460,7 +460,7 @@ public:
         std::optional<Hash> expectedCAHash = {});
 
     // FIXME: remove?
-    virtual StorePath addToStoreFromDump(const string & dump, const string & name,
+    virtual StorePath addToStoreFromDump(Source & dump, const string & name,
         FileIngestionMethod method = FileIngestionMethod::Recursive, HashType hashAlgo = htSHA256, RepairFlag repair = NoRepair)
     {
         throw Error("addToStoreFromDump() is not supported by this store");