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/libstore/store-api.hh

@@ -731,10 +731,6 @@ public:
 };
 
 
-/* Extract the hash part of the given store path. */
-string storePathToHash(const Path & path);
-
-
 /* Copy a path from one store to another. */
 void copyStorePath(ref<Store> srcStore, ref<Store> dstStore,
     const StorePath & storePath, RepairFlag repair = NoRepair, CheckSigsFlag checkSigs = CheckSigs);