This patch moves curve25519_keypair_t from src/or/onion_ntor.h to
src/common/crypto_curve25519.h, and adds new functions to generate,
load, and store keypairs.
The ntor handshake--described in proposal 216 and in a paper by
Goldberg, Stebila, and Ustaoglu--gets us much better performance than
our current approach.