* factor: Confine knowledge of num_traits to numeric::traits
This should make it easier to deal with API changes of num_traits,
or eventually switching to an abstraction provided in the stdlib.
- refactor internal version specifications to be ">=M.m.p" (where M.m.p is *already published*)
## [why]
Loosening internal version dependencies decreases the coupling between packages such
that packages can be published in a looser order. It allows the packages to be version
updated and published in tandem (ie, by using `cargo workspace ...`). Once published,
the internal versions can then be updated (again, to an *already published* package
version), as needed.
This avoids allocating on the heap when factoring most numbers,
without using much space on the stack.
This is ~3.5% faster than the previous commit, and ~8.3% faster than “master”.
