x86: sse3 != ssse3

Use the internal key stream buffer as the XChaCha temp key

Bring this up to date

Use `math/bits` on Go 1.9.

Use unaligned loads/stores for interacting with the state.

Fix the counter wrap detection for the IETF mode.

Minor amd64 assembly cleanups.

Add an explicit VZEROALL() call.

Remove a bunch of useless loops.

Move the hChaChaRef routine to where it makes more sense.

Check the CPUID max leaf before testing for AVX2 support.

Renamed one of the loops that I missed...

Fix `go vet` warnings.

Process 8 blocks at a time when doing AVX2.

Merge the AVX2 2 blocks and 1 block code paths.