Remove the primary key from the MultiIndex key specification

[maurolacy]

In MultiIndex, remove the primary key from the index key specification (i.e. when building / defining the `MultiIndex). That is, handle index key multiplicity internally, using the pk or another method.

This can be done by signalling the pk type with its own generic type (see #531), and then internally building an index key of type (IK, PK), that is, a tuple of Index key and Primary key. Of course, the index function must return (IK, PK) accordingly. (done by collapsing the ik, concatenated with the pk (#569)).

Alternatively, if the above is not possible make the index function type-aware on the pk element. That is, instead of using a bytes vector, use a proper type for the pk element. So that it's easier to handle internally. (not a good idea, as the pk is already in raw format at the indexes level)