Extract consts module (#282)

Extracts miscellaneous constants that were floating around in the
toplevel into their own module.

src/mscmap.rs

@@ -30,7 +30,7 @@
 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
-use crate::{piv::SlotId, serialization::*, Error, Result, YubiKey, CB_OBJ_MAX};
+use crate::{consts::CB_OBJ_MAX, piv::SlotId, serialization::*, Error, Result, YubiKey};
 use log::error;
 use std::convert::{TryFrom, TryInto};