new backend interface prototype

[borg323]

This is a prototype for the new backend interface, that doesn't have any dependence on the position encoding. For now only the search-cache interface is changed, the cache-backend interface is still the old one. The main changes to the interface are:

 // Check if entry is in the cache.
  bool CacheLookup(const PositionHistory& history, const MoveList& moves = {},
                   CachedNNRequest* entry = nullptr);
  // Adds a sample to the batch. Also calls EncodePositionForNN() if needed.
  void AddInput(const PositionHistory& history, const MoveList& moves);

The other change is that GetPVal() returns directly the 16 bit compressed float we used, now formalized to pfloat16:

// Returns compressed P value @move_id of @sample.
pfloat16 GetPVal(int sample, int move_ct) const;

[mooskagh]

Maybe still SmallArray to save 14 bytes per entry?