Add a test for ed25519 malleability and restore traditional behavior.

If an application really requires non-malleability, ED25519_PREVENT_MALLEABILITY
can be defined to enable the check.

This might become the default behavior depending on what other implementations
are planning to do.
Avoid a conditional jump
Test box_easy with overlapping in/out.
Add a test for chacha20poly1305 with overlapping pointers