Raise penalty blocked enemy pawn on 6th rank

Idea from Lyudmil Tsvetkov.

The value seems to be raised a bit abruptly, but as
Gary said, a blocked pawn on the sixth rank has been
instrumental in limiting king mobility in multiple
losses that I've seen from SF. A blocked pawn on fifth
rank is much less serious on the king safety impact.

Passed both STC
LLR: 2.97 (-2.94,2.94) [-1.50,4.50]
Total: 14551 W: 2750 L: 2607 D: 9194

and LTC
LLR: 2.96 (-2.94,2.94) [0.00,6.00]
Total: 43595 W: 6917 L: 6618 D: 30060

And even a retest at 60" fixed games 40K
ELO: 1.79 +-1.9 (95%) LOS: 97.0%
Total: 39889 W: 6018 L: 5813 D: 28058

bench: 7154916
Streamline implementation of Position::pretty()

Simpler and more in line with Bitboards::pretty()

No functional change.
Rise contempt when in advantage

This is a very discussed patch with many
argumentations pro and against. The fact is
it passed both STC:

LLR: 2.96 (-2.94,2.94) [-1.50,4.50]
Total: 16305 W: 3001 L: 2855 D: 10449

And LTC
LLR: 2.95 (-2.94,2.94) [0.00,6.00]
Total: 34273 W: 5180 L: 4931 D: 24162

Although it is true that a correct test should
include foreign engines, we commit it anyhow so
people can test it out in the wild, under broader
conditions.

bench: 7384368
Reset DrawValue[] before new search

Spotted by Ronald de Man

bench: 7384368
Revert "Generalize shift_bb() to handle double pushes"

Seems to intorduce some compiler warning as
reported by Gary. Warning seems bogus, but
revert anyhow.

No functional change.