Add feature flags to disable algorithms #315

I am using rustls-rustcrypto on embedded devices for https://github.com/n0-computer/iroh .

Binary size is a big problem on embedded devices, so I had to do a tiny patch to rustls-rustcrypto to feature flag each algorithm, so we can disable the ones we don't use.

It would be cool if there was some way to do this out of the box, then we wouldn't need a fork.