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/nix-store/xmlgraph.hh

@@ -4,6 +4,8 @@
 
 namespace nix {
 
-void printXmlGraph(const PathSet & roots);
+class StoreAPI;
+
+void printXmlGraph(StoreAPI & store, const PathSet & roots);
 
 }