hash: hide structure from header and make it internal

hash: hide structure from header and make it internal

rte_hash structure should not be a public structure,
and therefore it should be moved to the C file and be declared
as internal. rte_hash_hash implementation is also moved
to the C file, as it uses the structure.

This patch also removes part of a unit test that was checking
a field of the structure.

Signed-off-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>