KeyPair FromStr and Deserialize impls crash

[elsirion]

When trying to deserialize a KeyPair the thread crashes with

[libsecp256k1] illegal argument. rustsecp256k1_v0_6_1_ecmult_gen_context_is_built(&ctx->ecmult_gen_ctx)

this can be reproduced using the following test without special feature configurations by running cargo test test_broken_keypair_serde. I originally tested using serde, but could simplify it to FromStr.

    #[test]
    fn test_broken_keypair_serde() {
        let ctx = crate::Secp256k1::new();
        let keypair = KeyPair::new(&ctx, &mut thread_rng());
        let msg = keypair.secret_key().secret_bytes().to_hex();
        dbg!(&msg);
        let parsed_key: KeyPair = msg.parse().unwrap();
        assert_eq!(parsed_key, keypair);
    }

I'm not familiar enough with the low-level C API of libsecp to propose a fix keeping the unsafe code but would propose to use the global context instead and feature gate these impls accordingly.

[apoelstra]

Yikes. Yeah, these are basically completely broken without the global-context feature. We will need to backport this probably several versions back.

I think we should use the global context if available and otherwise have fallback code which constructs a signing context. It will be slow but probably better than not working at all, I think.

[apoelstra]

Wow, on further investigation it looks like I merged #313, which introduced this code, even though it had no tests and evidently never worked under any circumstances. All the discussion on that PR was about some unrelated terrible idea. What a process failure.