Eliminate the "store" global variable

Also, move a few free-standing functions into StoreAPI and Derivation.

Also, introduce a non-nullable smart pointer, ref<T>, which is just a
wrapper around std::shared_ptr ensuring that the pointer is never
null. (For reference-counted values, this is better than passing a
"T&", because the latter doesn't maintain the refcount. Usually, the
caller will have a shared_ptr keeping the value alive, but that's not
always the case, e.g., when passing a reference to a std::thread via
std::bind.)

src/libstore/local-store.cc

@@ -655,7 +655,7 @@ void LocalStore::checkDerivationOutputs(const Path & drvPath, const Derivation &
     assert(isDerivation(drvName));
     drvName = string(drvName, 0, drvName.size() - drvExtension.size());
 
-    if (isFixedOutputDrv(drv)) {
+    if (drv.isFixedOutput()) {
         DerivationOutputs::const_iterator out = drv.outputs.find("out");
         if (out == drv.outputs.end())
             throw Error(format("derivation ‘%1%’ does not have an output named ‘out’") % drvPath);
@@ -1335,7 +1335,7 @@ void LocalStore::registerValidPaths(const ValidPathInfos & infos)
            error if a cycle is detected and roll back the
            transaction.  Cycles can only occur when a derivation
            has multiple outputs. */
-        topoSortPaths(*this, paths);
+        topoSortPaths(paths);
 
         txn.commit();
     } end_retry_sqlite;