* feat: harden encoding/decoding functions against panics
Part of https://github.com/libp2p/go-libp2p/issues/1389
These kinds of functions:
1. Handle user input.
2. Often have out-of-bounds, null pointer, etc bugs.
3. Have completely isolated logic where local panics are unlikely to
cause memory corruption elsewhere.
* test: add a panic catcher test
The check was incomplete as it didn't test the curve. This switches us to use
basicEquals (which is also constant-time).
Note: This key type isn't used by anyone in-practice (to the best of my
knowledge).
