Extract consts module (#282)

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

src/msroots.rs

@@ -30,8 +30,11 @@
 // (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::{serialization::*, Error, Result, YubiKey};
-use crate::{CB_OBJ_MAX, CB_OBJ_TAG_MAX};
+use crate::{
+    consts::{CB_OBJ_MAX, CB_OBJ_TAG_MAX},
+    serialization::*,
+    Error, Result, YubiKey,
+};
 use log::error;
 
 const OBJ_MSROOTS1: u32 = 0x005f_ff11;