Fix incorrect 'ss' pointer in early stop check

The exclusion search used to verify one move is much
better than other shall be called with 'ss' and not
'ss+1'

No functional change.
Use alpha instead of beta-1

It is more directly related to a fail-low.

No functional change.
Retire psq_delta()

No functional change.
Introduce operator~(Piece c)

Small syntactic sugar to reverse piece color.

No functional change.
More consistent 'piece' variable naming

No functional change.
Convert pieceSquareTable to 3 dimensions

No functional change.
Zobrist::init() should be Position::init()

No functional change.
Don't need to expose namespace Zobrist

It can be local to position.cpp

No functional change.
Fix again early stop ss pointer

Fix was wrong becuase search starts from ss+1,
code is a bit tricky here, so rewrite in a way
to be more easy to read and understand.

Spotted by Eelco.

No functional change.
Don't use std::vector::data()

It is a C++11 only function.

Reported by Eelco.

No functional change.
Introduce Cut/All node definitions

Follow Don Dailey definition of cut/all node:

"If the previous node was a cut node, we consider this an ALL node.
The only exception is for PV nodes which are a special case of ALL nodes.
In the PVS framework, the first zero width window searched from a PV
node is by our definition a CUT node and if you have to do a re-search
then it is suddenly promoted to a PV nodes (as per PVS search) and only
then can the cut and all nodes swap positions. In other words, these
internal search failures can force the status of every node in the subtree
to swap if it propagates back to the last PV nodes."

http://talkchess.com/forum/viewtopic.php?topic_view=threads&p=519741&t=47577

With this definition we have an hit rate higher than 90% on:

    if (!PvNode && depth > 4 * ONE_PLY)
        dbg_hit_on_c(cutNode, (bestValue >= beta));

And an hit rate of just 28% on:

    if (!PvNode && depth > 4 * ONE_PLY)
        dbg_hit_on_c(!cutNode, (bestValue >= beta));

No functional change.
Reduce more CUT nodes

We got a good result from this tweak, in line with
what was already found by Don Dailey.

At short TC:
LLR: 2.95 (-2.94,2.94)
Total: 13097 W: 2742 L: 2598 D: 7757

At long TC:
LLR: 2.97 (-2.94,2.94)
Total: 7281 W: 1408 L: 1265 D: 4608

bench: 5108393
Reduce more CUT nodes only if parent node is reduced

So when we are doing a LMR search at the parent ALL node.

This patch didn't prove stronger at 60" TC
LLR: -2.97 (-2.94,2.94)
Total: 22398 W: 4070 L: 4060 D: 14268

But, first, it scores at 50%, second (and most important for me) the opposite,
i.e. normal reduction when parent node is not reduced, seems very bad:
LLR: -2.95 (-2.94,2.94)
Total: 7036 W: 1446 L: 1534 D: 4056

According to Don, this idea of increased reduction of CUT nodes
works because if parent node is reduced, missing a cut-off due to
reduced depth search (meaning position is somehow tricky) forces
a full depth research at parent node, giving due insight in this
set of sensible positions.

IOW if we expect a node to fail-high at depth n, then we assume it
should fail-high also at depth n-1, if this doesn't happen it means
position is tricky enough to deserve a research at depth n+1.

bench: 4687419
Fix description of TT entry

It was way outdated and wrong !

No functional change.
Revert "Reduce more CUT nodes only if parent node is reduced"

This reverts commit d54e8a59551a7ebcbf2c2155dce46c7386b1742a.

It was not proved with SPRT this tweak is stronger. So revert it
for now to follow fishtest guidelines.

bench: 5108393