numeric-contagion-constraint-back: better float contagion.
Derive that m is integer after (integerp (* m m)).
truncate constraint-propagate-back: handle negative divisors.
Propagate (nth-value 1 truncate)+typep.
Propagate unary-truncate+typep.
Propagate typep for integer truncate.
Unexport sb-posix:posix-fork.
Chain multiple constraint-propagate-back together.