argon2: add workaround for big 64-byte aligned allocations?

[newpavlov]

It was previously discussed in #566.

The claim is that for big allocations we can get higher performance by allocating len + 64 bytes with 1-byte alignment (we then can manually construct a 64-byte aligned region in the allocated memory with size len), than by directly allocating len bytes with 64-byte alignment.

cc @jonasmalacofilho