Stabilize field tactic

[CohenCyril]

Right now, the field tactic output nonzero side conditions in the form of a boolean conjunction [&& _ != 0, ... & _ != 0].
Ideally in order to produce a stable script, one should not rely on this exact shape, and prepare with have : _ != 0 statements before the call to the field tactic.

The ideal output of field would be individual subgoals of the shape _ != 0 instead.

E.g.

Lemma test x y : x != 0 -> y != 0 -> 1 / (x * y) = 1/x * 1/y.
Proof.
Fail by move=> xNZ yNZ; field.
by move=> xNZ yNZ; field; do ?[apply/andP; split].
Qed.