aes: restore fixslice64.rs

It appears that `fixslice64.rs` was accidentally overwritten with
`fixslice32.rs` in #208.

This restores the 64-bit implementation to what it was prior to that PR.

Closes #246.

Author: tarcieri

aes/src/soft/fixslice32.rs

@@ -1,5 +1,5 @@
-//! Fixsliced implementations of AES-128, AES-192 and AES-256
-//! adapted from the C implementation.
+//! Fixsliced implementations of AES-128, AES-192 and AES-256 (32-bit)
+//! adapted from the C implementation
 //!
 //! All implementations are fully bitsliced and do not rely on any
 //! Look-Up Table (LUT).