Reconsider `exportPrivateKey()`

[iaik-jheher]

exportPrivateKey() was mostly tacked onto the Signer interface as a hack, and I don't particularly like it. The vast majority of Signer implementations will implement this solely to have it return a "not exportable" failure.

I would strongly prefer if we moved it to a ExportablePrivateKey trait interface (or similar) that is only implemented by signers that actually can have exportable private keys; in particular, ephemeral signers.