The previous implementation exactly matched the behavior of libsodium 1.0.15
with the ED25519_COMPAT configuration, but this configuration wasn't used by
zcashd. This commit changes the validation rules to exactly match *without*
ED25519_COMPAT, and highlights the remaining inconsistencies with the Zcash
specification.
This ensures that any
```
impl From<T> for [u8; N]
```
is accompanied by a matching
```
impl AsRef<[u8]> for T
```
except for Signature, whose internals have to change (issue #9).