Before:
       gimli-hash:        120 MiB/s
       gimli-aead:        130 MiB/s

After:
       gimli-hash:        195 MiB/s
       gimli-aead:        208 MiB/s

Also fixes in-place decryption by the way.

If the input & output buffers were the same, decryption used to fail.

Return on decryption error in the benchmark to detect similar issues
in future AEADs even in non release-fast mode.