If you try to call PublicKey::from_secret() key with an incapable context it will
now return an error. Before it would pass through to the underlying library which
would terminate the process, something we strive to never expose.
Also change the from_ffi functions on various types to impl's of From to be more
Rustic. We cannot change the from_slice functions because they have error returns.
Also add a Secp256k1::without_caps() function which creates a capability-less
context. I find myself using this in so many places downstream that it seems
appropriate.
I didn't mean for both of these to go into the same commit, but given how
small the ECDH code was, and the fact that no commit prior to this one will
compile (as both libsecp256k1 and rustc have changed so much), I'm letting
it slide.
Thomas Eizinger
Andrew Poelstra