creusot-contracts generates many string warnings

[Lysxia]

warning: support for string types is limited and experimental

Aside from the handling of literals, I don't think strings are so peculiar that they deserve their own warning. In the end it's just a data structure like hashmap that needs someone to take the time to write down some extern specs.

[jhjourdan]

Agreed. We should remove this warning. There are so many things with "limited and experimental" support in Creusot that we should not emit warning for them.