Switch verify feature from ring to openssl?

[chifflier]

Followup of #69

Ring is a nice library, but it is not sure it is the best for our purpose: x509-parser needs to be able to verify all kind of cryptographic signatures (even old/deprecated/not ideal but correct algorithms). ring aims at providing a secure by default API, but our criteria is compatibility with previous implementations, so openssl may be a better choice.

[ReactorScram]

fwiw the new ring 0.17 cross-compiles easier, which is handy for our project, but apparently it needs a higher minimum Rust version 5132fdc

[cpu]

but apparently it needs a higher minimum Rust version

Indeed :-) There's some ongoing discussion in #148