this change removes the cryptopasta dependency.
a couple possible sources of problem with this change:
* the encoding used for ECDSA signatures on SignedMessage has changed.
the encoding employed by cryptopasta was workable, but not the same
as the encoding used for such signatures in the rest of the world
(most particularly, on ECDSA signatures in X.509 certificates). I
think we'll be best served by using one ECDSA signature encoding from
here on, but if we need to use the old encoding for backwards
compatibility with existing nodes, that can be arranged.
* since there's already a breaking change in SignedMessage, I changed
it to send and receive public keys in raw PKIX format, instead of
PEM. PEM just adds unhelpful overhead for this case.
