add flags for openssl-only crypto

[connor4312]

Fixes #50

This introduces an on-by-default rs-crypto flag, which enables the existing Rust-based crypto libraries (including aes and ED25519). However, these implementations can be removed by disabling the flag. If it's disabled, then openssl (when turned on) will stand in for them, in a less performant way.

Note that while OpenSSL 3.x does have some ED25519 support, I have not done the work to make that compatible as well--partly because ED25519 is not yet an approved algorithm for my company to use, and partly to retain compatibility with OpenSSL 1.x

[connor4312]

I will fix up the build/tests in the morning, it's late here already (but it works!)

[Eugeny]

:+1: merge c1472f6 for CI setup

[connor4312]

Hmm, not sure what's up with the build failure; that test works on my machine -- though later doctests fail since a lot of them show examples with ed25519 keys... Not sure whether I'll have time to dive into that more today or not

[Eugeny]

No worries and no rush