Merge branch 'ci/ralf/warnings' into 'master'

fix or silence Coq 8.10 warnings

See merge request !52

_CoqProject

 -Q theories stdpp
+# "Declare Scope" does not exist yet in 8.9
+-arg -w -arg -undeclared-scope
 theories/base.v
 theories/tactics.v
 theories/option.v

theories/numbers.v

@@ -399,7 +399,7 @@ Next Obligation. intros x y; apply Qclt_not_le. Qed.
 Next Obligation. done. Qed.
 Program Instance Qc_lt_dec: RelDecision Qclt := λ x y,
   if Qclt_le_dec x y then left _ else right _.
-Solve Obligations with done.
+Next Obligation. done. Qed.
 Next Obligation. intros x y; apply Qcle_not_lt. Qed.
 
 Instance: PartialOrder (≤).