lib: Add hash and use const a bit more intelligently

lib: Add hash and use const a bit more intelligently

This commit adds code to notify the compiler that we
will not be changing the arguments to nexthop2str
and we expect thre return to be treated the same.

Additionally we add some code to allow nexthops to
be hashed to be used in a hash.

Signed-off-by: Quentin Young <qlyoung@cumulusnetworks.com>