Eliminate seed from XXH32_state_s

[stbrumme]

The seed variable is stored in the XXH32_state_s struct because it's used when XXH32_digest_endian() is called after less than 16 bytes. However, that means that state->v1...v4 are is still unmodified and especially state->v3 has it's initial value which was state->seed.
If you replace line 665 by "h32 = state->v3 + PRIME32_5;" then we can get rid of state->seed.

See my xxHash implementation at http://create.stephan-brumme.com/xxhash/ (written from scratch) for a proof-of-concept.

I haven't analyzed xxHash64 yet, but after a quick look at your code it seems we could apply the same trick.

[Cyan4973]

Thanks @stbrumme, this is an excellent idea.

While it's certainly possible to implement it, I'm nonetheless worried about a possible ABI break when reducing structure size.
A bad scenario would be as follows :

• Program uses the new API/ABI with smaller structure size. It allocates enough space for the new structure.
• It then links to a library which expects the older, larger, size. During initialization, library writes / read beyond allocated buffer. Segfault.

This scenario is supposed to be explicitly forbidden in xxhash.h,
but it's difficult to guarantee everyone follows the rules.

So a cautious transition scenario could be as follows :

• Implement your idea, remove seed, to be replaced by state->v3 instead.
• Preserve the structure size, by adding a "reserved" field at the end
  • Make sure the "reserved" field is never read nor written within library
• Sometimes in the future, reduce structure size, by removing the "reserved" field

It should reduce risks in case of version mismatch, by allowing an "overlap" period, where the structure is compatible with older implementations, and implementation is compatible with future structure.

For discussion

[Cyan4973]

Partly implemented in v0.6.2.
Tail space in now "reserved", neither read nor written.
Should be possible to remove it completely in a few versions.