Add a wrapper around, and test and build support for, curve25519.

We want to use donna-c64 when we have a GCC with support for
64x64->uint128_t multiplying.  If not, we want to use libnacl if we
can, unless it's giving us the unsafe "ref" implementation.  And if
that isn't going to work, we'd like to use the
portable-and-safe-but-slow 32-bit "donna" implementation.

We might need more library searching for the correct libnacl,
especially once the next libnacl release is out -- it's likely to have
bunches of better curve25519 implementations.

I also define a set of curve25519 wrapper functions, though it really
shouldn't be necessary.

We should eventually make the -donna*.c files get build with
-fomit-frame-pointer, since that can make a difference.