Remove RecInt::rmint

[Breush]

Rationale is that:

• Basically, rmint<K> is a Modular<ruint<K>,ruint<K+1>>, and should behave similarly (if they don't, we should specilized the Modular one);
• There are two variants rmint<K, MG_INACTIVE> and rmint<K, MG_ACTIVE> that have underlying montgomery variant active or not. The second one should be equal to Montgomery<ruint<K>>, with specialization if needed for performances;
• rmint<K> are storing modules as static static ruint<K> p which means that you cannot have two rmints with different modules at the same time (!);
• rmint are breaking the current design of Givaro by having no Field class.

[ClementPernet]

I think the last 2 points are strong enough to motivate this change. I kind of remember discussing this with @jgdumas who had a good argument for the existence of this design. Let's wait for his feedback.

[jgdumas]

rmint could be replaced by StaticElement<Modular<ruint>>, so seems good to go.