Simplify return type of FloatDomain predicates

[sim642]

While looking at all the Unknown cast kinds in #1896, I realized that all the int casts of FloatDomain predicate results are pointless because they are already int by construction.

In order to enforce this on the OCaml type level, it's better to have those predicates return bool option instead of ID.t, which is implicitly assumed/required to correspond to a boolean (but in int, not _Bool).
In fact this revealed that some of those operations actually were incorrectly returning _Bool, so the cast was necessary before.

This simplifies their implementations and reduces the IntDomain dependency of FloatDomain.
I've been thinking about a similar simplification in IntDomain, but it's a bit more involved due to the many different module interfaces.