* Added support to read openssh public keys and encrypted private keys
* reworked private key handling
* made password the default; generating keys without password
requires explicit "--no-password"
* use protobuf for encryption-header
* use fixed size file-header (42 bytes) before the encryption-header
* add encryption/decryption contexts
* teach MakePrivateKey() to fixup its internal public key bits
* added encrypt, decrypt commands (empty now)
* use command abbreviation util library
* switched to go1.13 (ed25519 now in stdlib)
* added new code to convert/manage Ed25519 keys to Curve25519
(thanks to FiloSottile/age)
