Update ed25519 library: performance optimizations

Signed-off-by: Sergey Matveev <stargrave@stargrave.org>

diff --git a/src/github.com/agl/ed25519 b/src/github.com/agl/ed25519
index d2b94fd789ea21d12fac1a4443dd3a3f79cda72c..278e1ec8e8a6e017cd07577924d6766039146ced 160000 (submodule)
--- a/src/github.com/agl/ed25519
+++ b/src/github.com/agl/ed25519
@@ -1 +1 @@
-Subproject commit d2b94fd789ea21d12fac1a4443dd3a3f79cda72c
+Subproject commit 278e1ec8e8a6e017cd07577924d6766039146ced