Statically enforce the size of PrivateKey.{zero_values, one_values}

[romac]

As @nvesely found out in #12, we are currently not enforcing the length of the one_values and zero_values fields of the PrivateKey structs.

If one_values: Vec<Vec<u8>> is different in len() than zero_values, the
code can either panic, or it's possible different PrivateKeys are evaluated
as equal. In both these examples, at least one key would be of invalid
structure, but there's nothing about the PrivateKey struct that enforces
valid structure in this sense.

Perhaps this is an argument to create key-length-specific types*.

* E.g.,

pub struct PrivateKey512 {
    zero_values: [[u8; 512 / 8]; 512],
    one_values: [[u8; 512 / 8]; 512],
    // ...
}