StorePath: Rewrite in C++

On nix-env -qa -f '<nixpkgs>', this reduces maximum RSS by 20970 KiB
and runtime by 0.8%. This is mostly because we're not parsing the hash
part as a hash anymore (just validating that it consists of base-32
characters).

Also, replace storePathToHash() by StorePath::hashPart().

src/libutil/rust-ffi.hh

@@ -1,4 +1,5 @@
 #pragma once
+#if 0
 
 #include "serialise.hh"
 
@@ -185,3 +186,4 @@ struct Result
 };
 
 }
+#endif