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 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.
