Expose residue params and modulus in DynResidue

[fjarri]

The functionality I would like to have:

• extract DynResidueParams from a DynResidue object (to be able to, given one residue, initialize another one with the same params);
• extract the modulus from DynResidueParams (to be able to implement some modulus-depending algorithms, e.g. square root)

Edit: I decided to wait on exposing the Montgomery form for now; I will just retrieve before serializing. Exposing it exposes the specific representation used by the residue, and the API needs some thought, e.g. whether we do a range check when assembling a DynResidue from the pre-calculated Montgomery form and params.

Changes:

• Added DynResidueParams::modulus()
• Added DynResidue::params()