types: fix type aliasing violation
types: fix type aliasing violation

Any pointer can be casted to char*, but not the other way around. This
causes GCC6 to misoptimize timestamp_type_impl::from_string().

Signed-off-by: Paweł Dziepak <e29bb3265626c60caf76d587cabdfcd8b379ef80@scylladb.com>
Message-Id: <1468413349-27267-1-git-send-email-e29bb3265626c60caf76d587cabdfcd8b379ef80@scylladb.com>

duarten/scylla,scylladb/scylla,kjniemi/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,duarten/scylla,avikivity/scylla,raphaelsc/scylla,avikivity/scylla,scylladb/scylla,duarten/scylla,kjniemi/scylla,kjniemi/scylla,raphaelsc/scylla,raphaelsc/scylla