`thread` provides insufficient expected type info to its thunk argument

[samth]

Since the change to add keyword arguments to thread (racket/racket#5304), programs like this no longer type check:

(thread
  (lambda ()
    (let loop () ; note no annotation
       (loop))))

This led to failing drdr tests but also to failures in the digimon package. This will need to be fixed somehow before the release with parallel threads.

[samth]

Fixed by #1465