Bring changes from SUPERCOP 2014-04-25 to ed25519/ref10
Sync stream_salsa20_amd64_xmm6.S with SUPERCOP 2014-04-25
Sync salsa20/ref with supercop-20140425
But these changes may be reverted unless there is a solid reason for
adding these copies to the stack.
Add overdue crypto_sign test
Make it clear that crypto_sign_SECRETKEYBYTES actually stands for 2 * 32