compress_edwards() is now named compress() and works only on points
which are in Edwards form. Similarly, compress_montgomery() is now
also called compress(), and it only works on point already in
Mongomery form.
To switch between forms, use to_montgomery().
Conversion from Montgomery directly to Edwards is not yet implemented.
* CHANGE the API requested in
https://github.com/isislovecruft/curve25519-dalek/issues/47,
hopefully for the better.
Having _BASEPOINT_TABLE and _BASEPOINT_POINT means that it's not possible to
use the slow, generic scalar mult in place of the fast, precomputed scalar
mults.
