There is an unexpected function ZZp, which can choose among 3 ZZ/p implementations.
A few changes that are needed for the ZZp change last commit.
Added new template type DenseMatrixDef

which allows to switch between generic coeffs and flint, etc.
Changed internals of dmat.  WARNING: routines which modify mat size do not work yet.
Added flint and ffpack new functions for set, rank, inverse.

Code is not clean yet though, and some mutable matrix code
involving changing matrix size, is not working yet.