The implementation of `Keystore` requires a particular layout and naming
of keys. This is a rather strong limitation. Lift it and allow
specifying an arbitrary path for the secret and public key.
As it is possible to derive the public key from the secret key, make
the public key in `Keystore` optional. Also allow constructing
`MemorySigner` can now be initialized without a public key, by
deriving it.
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