Optionally support openssl instead of boringssl backend

[niklasf]

As another workaround for the linker conflict with openssl that doesn't sacrifice as much performance.

$ cargo bench
[...]
rsa-2048 - sign: 1.81 K/s
rsa-2048 - verify: 72.28 K/s

$ cargo bench --no-default-features --features pure-rust
[...]
rsa-2048 - sign: 714.39 /s
rsa-2048 - verify: 6.75 K/s

$ cargo bench --no-default-features --features openssl
[...]
rsa-2048 - sign: 1.29 K/s
rsa-2048 - verify: 41.05 K/s

[jedisct1]

BoringSSL and OpenSSL have different features and APIs. Having to support both would require constant, additional work with little benefits.

Instead of patching each crate using either the former or the latter, your Rust skills would be better utilized in addressing the root cause, that affects far more that jwt-simple: cloudflare/boring#197

[niklasf]

Fair enough. One more idea for this specific crate: Would you be open to making RSA optional, since it's the only problematic algorithm (for this issue, but also portability in general) and others may be preferred in any case?

[jedisct1]

I'd like to eventually use boringssl for all the crypto, since API stability, performance and security are much better than the rust implementations (including mine 😂).