Matthew suggests that using exceptions for failure
in the implementation of `subtype` is a performance
problem. This commit removes all use of exceptions
for failure in subtype.rkt, replacing it with the
standard Racket #f/value option.
Extensive use is made of the `subtype-seq` form,
which is basically do-notation for the subtyping
monad.
(cherry picked from commit a6f110893f)
20f546110b