Be more optimistic in aspiration window

Be more optimistic wrt search instability, and set the unviolated bound
half window.

STC
LLR: 2.96 (-2.94,2.94) [-1.00,4.00]
Total: 16362 W: 3371 L: 3197 D: 9794

LTC
LLR: 2.94 (-2.94,2.94) [0.00,5.00]
Total: 21666 W: 3780 L: 3569 D: 14317

Bench: 6807896

Resolves #105
Codestyle massage Search::init()

* remove some erroneous comments, that were based on the ONE_PLY == 2.
* rename hd to d, because there's no more half-depth in SF.
* rescope variables into the for loops.
* reindent the for loops correctly.
* add a comment to explain the eval improving part (not so obvious to read
this code as array has 4 dimensions).

No functional change.
Introduce distance() and unify some API

Original work by Lucas.

No functional change.
Assorted code-style triviality

No functional change.
Retire pvMove in search()

Now we can directly replace it with
the definition resulting in simpler
and possibly faster code because
PvNode is evaluated at compile time.

No functional change.
Fix bounds of FutilityMoveCounts

This is a left-over from ONE_PLY == 2.

No functional change.

Resolves #107