Convert Ed25519 and Curve25519 functions to plain C

include/olm/account.hh

include/olm/crypto.h

include/olm/crypto.hh

include/olm/pickle.hh

include/olm/ratchet.hh

include/olm/session.hh

include/olm/utility.hh

src/account.cpp

src/crypto.cpp

src/olm.cpp

src/pickle.cpp

src/ratchet.cpp

src/session.cpp

src/utility.cpp

tests/test_crypto.cpp

tests/test_ratchet.cpp

tests/test_session.cpp