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.hh

@@ -303,6 +303,10 @@ private:
 
     int openGCLock(LockType lockType);
 
+    void findRoots(const Path & path, unsigned char type, Roots & roots);
+
+    void findRuntimeRoots(PathSet & roots);
+
     void removeUnusedLinks(const GCState & state);
 
     void startSubstituter(const Path & substituter,