RUSTSEC-2021-0146: Crate `twoway` deprecated by the author

[github-actions]

Crate twoway deprecated by the author

Details
Status	unmaintained
Package	twoway
Version	0.1.8
URL	https://github.com/bluss/twoway
Date	2021-05-20

The commit e99b3c7 releasing version 0.2.2 explicitely deprecates twoway in favour of memchr crate.

See advisory page for additional details.

[rillian]

This is a dev-dependency, so not urgent. The crate is a transitive dependency of the warp web service framework we use for the example ppoprf server.

Possible fixes:

• Upstream merges the recommended fix and publishes a new release.
• We bump MSRV and switch to a different web framework.
• We rewrite the example in lower level code to drop the framework dependency.
• We drop the server example entirely.

[rillian]

This was addressed by a warp update, probably #236