Iterator optimizers can provide type information for public bindings

Iterator optimizers are now allowed to provide type information for public bindings. For instance

(for ((i (in-naturals :bound 10 :fixnum t)))
  ...)

will, normally, implicitly declare i to be of type fixnum. This can be disabled if you really must assign values to iterator variables which are of other types.

The only standard iterator optimizer which actually does this is in-naturals at the moment.