Use ::core in lieu of ::std, allowing this crate to be usable in #![no_std]
environments.
Gates features that presently depend on ::std (presently just rand) behind a
"std" cargo feature, which is enabled by default.
Would be good to have the basepoint itself, but this doesn't seem possible: the
fields of an ExtendedPoint are not public, so we can't construct them outside
of curve.rs (by design, to prevent constructing invalid points). Maybe const
fn support could change this.
This should be brought back later as part of reworking the compressed
point formats / serialization code. Right now there's just
"CompressedPoint" which is in the ed25519 format. Ideally, users should
be able to serialize points to formats used for X25519, for decaf, etc.
and not have to worry too much about the internal model.
