Alternative take on #153: fix `le` in future versions of Coq

This should provide compatibility for https://github.com/coq/coq/pull/12162

Other changes:

• Use Arith instead of NPeano, since the latter is deprecated. While it may also be possible to import NPeano, numbers export PArith NArith ZArith, so it seemed logical to also export Arith.
• Add test to check for correct version of le.

tests/numbers.ref

+le
+     : nat → nat → Prop
+le
+     : nat → nat → Prop
+le
+     : nat → nat → Prop
+le
+     : nat → nat → Prop