Prevent caching of BACK-EDGE instances in SPECIFIER-TYPE

BACK-EDGE instances appear as intermediate results. Prevent them from
being cached.
Handle (pairs of) recursive types in TYPE=

Keep track of seen pairs of types and return true for seen pairs: if
such a pair turns out to be TYPE= in every other regard, it is TYPE=.
Added TWICE
Indentation of IR1-TRANSFORM-TYPE-PREDICATE
cosmetic changes in SOURCE-TRANSFORM-TYPEP
Handle recursive /values/ in SOURCE-TRANSFORM-TYPEP
TODO temp commit: debugging stuff
TYPE-SPECIFIER tests
Support recursive values in %%TYPEP
Support recursive types in CONS-TYPE-MIGHT-BE-EMPTY-TYPE
Support recursive types in CONS-TYPE-SIMPLE-INTERSECTION2-HELPER
(very unfinished)
TYPEP tests