Added strings to prelude to fix printing of strings.length

This should hopefully fix strings.length showing up in random places. I tested it with iris and RefinedC. Both compile without changes and length is no more printed as strings.length in RefinedC. See the comment for the reasoning why I think this works.

theories/prelude.v

@@ -10,4 +10,13 @@ From stdpp Require Export
   fin_sets
   listset
   list
+  (* This must be exported after base (which exports Coq.List) since
+  it exports the length notation to ensure that we use List.length and
+  not Strings.length. If we don't export strings here and some file
+  imports strings and then the prelude (which seems to happen with
+  Iris), the definition in Coq.List will shadow the notation in
+  stdpp.strings and length will be printed as strings.length since Coq
+  prefers printing the notation (strings.length) instead of the
+  definition which shadows it. *)
+  strings
   lexico.