Squashed 'src/ed25519-donna/' content from commit 04223b0

git-subtree-dir: src/ed25519-donna
git-subtree-split: 04223b04e22f5eff32c6c27e25194d4d984c6f41
Merge commit 'd229e84ed3a682ced464d9e39146044ac80a7989' as 'src/ed25519-donna'
SConstruct:  Add ed25519.c
SerializedTransaction:  Combine checkSign() overloads

Callers don't need to specify the signing key -- they're just retrieving
the key from the SerializedTransaction and then passing it back.

This simplifies Ed25519 implementation.
RippleAddress:  Return Blob from accountPrivateSign()
RippleAddress:  Pass full message for signatures

Ed25519 signatures require the entire message, not just a 256-bit hash.