Use graviola as a TLS backend

[yorickpeterse]

Description

ring is no longer maintained, and while there's a discussion about transferring ownership it seems not much development is likely to happen in the future.

graviola is an alternative that promises to be much easier to deal with in terms of compilation and distribution, and based on their benchmarks performs quite well compared to ring (sometimes better, sometimes worse).

The tls-graviola branch contains a bunch of changes needed to make this happen, but this work is blocked by the following:

• Invalid certificates are not signaled as such when used through rustls ctz/graviola#67
• Adding graviola as a dependency introduces some dependencies with multiple versions, most of which are introduced through the rand crate, which I want to get rid of as part of Replace usage of the rand crate with a custom ChaCha12 CSPRNG #813

Related work

No response