* Rename `signing` and `verifying` modules
Renames the following modules:
- `keypair` => `signing`
- `public` => `verifying`
Renaming these in an individual commit preserves the commit history.
This is in anticipation of renaming the following per #225:
- `Keypair` => `SigningKey`
- `PublicKey` => `VerifyingKey`
* Rename `Keypair` => `SigningKey`; `PublicKey` => `VerifyingKey`
As proposed in #225, renames key types after their roles:
- `SigningKey` produces signatures
- `VerifyingKey` verifies signatures
The `SecretKey` type is changed to a type alias for `[u8; 32]`, which
matches the RFC8032 definition:
https://www.rfc-editor.org/rfc/rfc8032#section-5.1.5
> The private key is 32 octets (256 bits, corresponding to b) of
> cryptographically secure random data.
