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.cc

@@ -1,3 +1,4 @@
+#if 0
 #include "logging.hh"
 #include "rust-ffi.hh"
 
@@ -20,3 +21,4 @@ std::ostream & operator << (std::ostream & str, const String & s)
 }
 
 }
+#endif