Remove Default impls with unimplemented!

[conradoplg]

Using DefaultIsZeroes requires us to implement Default on SpendAuth and Binding, but those are supposed to be unconstructable. This was solved by adding a Default impl which calls unimplemented! but that doesn't seem ideal.

Check for an alternative and remove the Default impls. One possibility is to explicitly implement Zeroize and Drop. See https://github.com/ZcashFoundation/redjubjub/pull/87/files#r628834530 and ZcashFoundation/redjubjub@2745d37 for context.