Use `Integer::div_floor` explicitly (#694)

* Use `Integer::div_floor` explicitly

rust-lang/rust#88581 adds several new integer methods as inherent impls. These new methods are a breaking change accepted as a minor change. They already cause build failures with nightly and will eventually cause build failures with stable as well, unless rust-lang changes course. This uses `Integer::div_floor` explicitly to avoid accidentally calling the new nightly methods, same as done for rust-num/num-bigint#218.

* run cargo fmt

Co-authored-by: Russell Cohen <rcoh@amazon.com>