Upgrades the edition, with changes primarily performed by
`cargo fix --edition`.
In order to keep the changes to a minimum, this doesn't include
automated idiom fixes (i.e. `cargo fix --edition-idioms`).
This is needed to unblock the edition upgrade for the RSA crate:
https://github.com/RustCrypto/RSA/pull/135
The *idea* of `big_digit` and its type aliases is that we may someday
use something other than `u32` in the representation, perhaps even
different sizes for different targets. That's still a possibility, but
I think it's not really feasible to expose this variation in the public
API. Calling `BigUint::from_slice([1, 2, 3])` is only meaningful if you
know what that size is, and users can't really alternate this kind of
thing based on a type definition. So for now, we just commit to `u32`
units in the public API, no matter what we may do internally.
This removal is a breaking change, part of the 0.2 semver bump. If I'm
wrong and somebody can show a compelling use case for `big_digit`, we
can always add things back into the public API later.
The test modules were getting huge, and some of its functions were
actually a huge amount of code due to macros, causing tests to take a
long time just to compile. They are now separated into a few different
tests, and the scalar macros especially are now expanded more sparingly
in just a few `check()` functions.
Test compile times for me went from about 25 seconds to 1.5s in debug
mode, and from 300 seconds (!) to about 8s in release mode.
