Small bonus to favor thorn pawns

[snicolet]

We increase a little bit the midgame value of pawns on a4, h4, a6 and h6.

Original idea by Malcolm Campbell, who tried the version restricted to the
pawns on the H column a couple of weeks ago and got a patch which almost
passed LTC. The current pull request just adds the same idea for pawns on
the A column.

Possible follow-ups: maybe tweak the a5/h5 pawn values, and/or add a malus
for very low king mobility in midgame?

STC:
LLR: 2.94 (-2.94,2.94) {-0.50,1.50}
Total: 33416 W: 6516 L: 6275 D: 20625
Ptnml(0-2): 575, 3847, 7659, 4016, 611
https://tests.stockfishchess.org/tests/view/5ee6c4e687586124bc2c10d4

LTC:
LLR: 2.95 (-2.94,2.94) {0.25,1.75}
Total: 134368 W: 16869 L: 16319 D: 101180
Ptnml(0-2): 908, 12083, 40708, 12521, 964
https://tests.stockfishchess.org/tests/view/5ee74e60aae8aec816ab756a

closes #2747

Bench: 4263543

[vondele]

nice to see that this passed. Good timing as well, kind of confirms that the quantization patch doesn't interfere with small eval changes.

[xoto10]

There is also a related test, https://tests.stockfishchess.org/tests/view/5ee96340563bc7aa756002e3 that looks likely to go to ltc.
We might want to consider combining the changes?

Edit. My mistake, the test named already includes this change. And looks worse than this one, at least so far.

[vondele]

Thanks!