Encourages error checking in mikmeta.

This interface makes it much more clear that error checking is necessary on
calls to mikmeta_deserialize. Especially when the mikmeta object is expected
from a function which hides a call to mikmeta_deserialize.
Adds fragment fetching utilities.

These fragment fetching functions make the mikpack code more clear, shorter, and
less error prone than doing addition with the offset and header constants.