Initial support for key certificates and arbitrary types and lengths

of signing keys and signatures in RouterIdentities and Destinations.
Untested, not even for regressions, except with command line
using PrivateKeyFile.
Based on preliminary spec at http://zzz.i2p/topics/1442?page=1#p7524
Not done:
 - Transport handshake signing
 - Configuration of default type
 - Specification of type in options to I2PSocketManagerFactory
 - Specification of type in i2ptunnel
 - Fix up caching of SigningPublicKey and P256 key cert
 - Any non-default crypto type in the key cert
 - Documentation