sleep: Add more precautions to avoid consing on x86.
Fix NCONC type derivation.

Properly check the types of arguments, instead of testing for subtypes
or supertypes of LIST, check for arguments to be subtypes of NULL or CONS.

Reported by Jerry James.
fixes in EXPT type derivation

It was possible to construct mostly (but not completely) unobservable
bogus floating-point types when deriving the type of functions
returning the value of calls to EXPT.  Noticed by Vsevolod Dyomkin,
who found a way to observe it by redefining methods.
Fix APPEND/NCONC type derivation properly this time.

Use type-intersection for checking types, it's more robust than what
was there before.

And a slight improvement. When argument in the middle can't be a NIL,
then the end result is guaranteed to be a CONS. Previously, the
assumption was if the type is a CONS, but that doesn't work with types
like (or cons vector).