Add missing Bind Scope for fin

This allows for automatic declaration of fin_scope when in a function argument of type fin n. For example in a defined function f {n}: fin n → T, you can now do f 45 instead of f 45%fin (assuming the n can be deduced of course). This might be a breaking change in very weird circumstances, but it otherwise allows a more convenient use of the fin notations.

stdpp/fin.v

@@ -17,6 +17,7 @@ Notation FS := Fin.FS.
 
 Declare Scope fin_scope.
 Delimit Scope fin_scope with fin.
+Bind Scope fin_scope with fin.
 Global Arguments Fin.FS _ _%fin : assert.
 
 (** Allow any non-negative number literal to be parsed as a [fin]. For example