* 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
In practice, this is impossible to exploit without being able to corrupt the
private key which would allow a much simpler guess-and-check attack. However,
it's still a bad practice to compare private key material like this.
