Swap import of Peano and Utf8 to ensure that Utf8 notations are preferred.

Coq PR #12950, among others changes, gives to import the effect of reactivating the imported notations.

This has an impact for stdpp, e.g. on printing le n m as either m <= n or m ≤ n, due to the order of imports between Utf8.v and Peano.v in base.v.

The Coq PR is still at the beginning of a process of discussion but the change to stdpp is backward compatible (as far as I can judge) and it should be safe to merge it anyway.