read circularities and type declarations

[Nikodemus Siivola]

BUGS #410

Consider the definition
    (defstruct foo (a 0 :type foo))
followed by
    (setf *print-circle* t) ; just in case
    (read-from-string "#1=#s(foo :a #1#)")
This gives a type error
  #S(SB-IMPL::SHARP-TAG :LABEL 1 :VALUE SB-IMPL::+SHARP-MARKER+) is not of type FOO
because of the implementation of read circularity, using a structure as a marker for the previously-referenced object.

[Nikodemus Siivola (nikodemus)]

Downgrading to low.