Support flexible parameters

[psivesely]

One should be able to generate a validated parameters struct to specify n, m, and w. Since there are some restrictions on these values (e.g., 1 <= n <= 512), the new method of this struct should validate these parameters.

If the parameters are flexible, they'll need to be encoded in the public key.

I think it may be best to put the global secret seed and the public seed in a new struct together with this parameter struct, which could mean generating an object containing all values needed for signing at once. Maybe this is actually what the SecKey struct should be, and then the Addr struct (currently just a type alias actually) should include a used bool.