Change Debug implementation for SecretKey/SharedSecret

[rustaceanrob]

The only purpose of hashes in this crate is to blind secret keys when implementing Debug. By using a different blinding function, particularly one in ffi, we may remove hashes entirely. Notably, since there is no longer a feature requirement, all users would get a Debug implementation with a finger print.

I have more questions than I do solutions. Why is this function pointer an Option? When getting the nonce, how do we want to display it? Does the message parameter matter?

[rustaceanrob]

Didn't see #837

[apoelstra]

Hah, oops! I guess we were working at the same time on this.

I have more questions than I do solutions. Why is this function pointer an Option?

Because the Option version represents the type that gets passed to the C FFI (since C function pointers may be NULL). The intention of this function is that it's passed to the C FFI, not that it be called directly.

When getting the nonce, how do we want to display it?

I just took the first 16 hex characters (64 bits), which matches the previous code. This gives us 32 bits of collision resistance which should be fine for any sane use of Debug output.

Does the message parameter matter?

No. Basically, you want to somehow stick (a) the secret, (b) the word "debug" somewhere, and everything else is just padding.