Extend Rust FFI

Do idiomatic C++ copy and move constructors for a few things, so
wrapping structs' defaults can work.

src/libstore/store-api.cc

@@ -687,21 +687,6 @@ void copyClosure(ref<Store> srcStore, ref<Store> dstStore,
 }
 
 
-ValidPathInfo::ValidPathInfo(const ValidPathInfo & other)
-    : path(other.path.clone())
-    , deriver(other.deriver ? other.deriver->clone(): std::optional<StorePath>{})
-    , narHash(other.narHash)
-    , references(cloneStorePathSet(other.references))
-    , registrationTime(other.registrationTime)
-    , narSize(other.narSize)
-    , id(other.id)
-    , ultimate(other.ultimate)
-    , sigs(other.sigs)
-    , ca(other.ca)
-{
-}
-
-
 std::optional<ValidPathInfo> decodeValidPathInfo(const Store & store, std::istream & str, bool hashGiven)
 {
     std::string path;