Moved rs_hash implementation into public interface

This patch moves the implementation of the rs_hash type into the public
interface. The helper functions for handling the hash type are still in
the internal utilities, but moved from

    src/libreset/

into

    src/libreset/util/

so we can include "libreset/util/hash.h" in the source when including
the internal utility and "libreset/hash.h" when including the hash type
implementation from the public interface.