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/download.cc

@@ -188,7 +188,7 @@ DownloadResult downloadFile(string url, const DownloadOptions & options)
 }
 
 
-Path downloadFileCached(const string & url, bool unpack)
+Path downloadFileCached(ref<StoreAPI> store, const string & url, bool unpack)
 {
     Path cacheDir = getEnv("XDG_CACHE_HOME", getEnv("HOME", "") + "/.cache") + "/nix/tarballs";
     createDirs(cacheDir);