Migrate to Rust edition 2021

update code for the Rust 2018 edition

_Play-RSA_ is a Rust implementation of RSA cryptography for
pedagogical use.

The first version of _Play-RSA_ was published in 2015. The basic
algorithms have not changed since, only the Rust syntax changed slightly
with the Rust-2018-edition. This release updates the code and
the library dependencies.

[play-rsa] is available for download on Github.