Add parallel search with shared transposition table

[vinc]

See #8

[vinc]

SharedTranspositions is a bit of a hack because it implements Sync without making sure that the inner UnsafeCell<Transpositions> is thread safe. But it allows Arc<SharedTranspositions> to share the transposition table between threads.

An improvement would be to only share entries of Transposition, so that all threads can access the same Box<[Transposition]> and we could then put some sort of lock on Transpositions#set.

[vinc]

The build fail on rust 1.20.0 but should pass on the next stable version.

[vinc]

Rust 1.21.0 is now stable and it fixes compile errors about Clone not implemented for big arrays.

[vinc]

The search threads need a stack size of at least 4 MB so the default at 2 MB had to be increased to fix stack overflow.

[vinc]

Something is not yet right with this PR, increasing the number of threads doesn't make the engine any faster, and it's even weaker on one thread when it shouldn't be.

$ cutechess-cli \
  -engine cmd=littlewing-v0.3.0-26-gfab2928 \
  -engine cmd=littlewing-v0.3.0-18-g441470e \
  -each proto=xboard tc=40/10+0.1 \
  -sprt elo0=0 elo1=5 alpha=0.05 beta=0.05 \
  -openings file=2moves.pgn -repeat -rounds 10000 \
  -pgnout games.pgn -concurrency 8
...
Score of Little Wing v0.3.0-26-gfab2928 vs Little Wing v0.3.0-18-g441470e: 338 - 501 - 168  [0.419] 1007
Elo difference: -56.74 +/- 19.79
SPRT: llr -2.96, lbound -2.94, ubound 2.94 - H0 was accepted
Finished match

[vinc]

The bugs have been removed and the code cleaned up, but more work is to be done on the SMP code to see any positive results with more threads.

Score of Little Wing v0.3.0-31-ge3629c2 vs Little Wing v0.3.0-18-g441470e: 508 - 503 - 295  [0.502] 1306
Elo difference: 1.33 +/- 16.57
SPRT: llr -0.0817, lbound -2.94, ubound 2.94
Finished match

Score of Little Wing v0.3.0-33-g2b35191 vs Little Wing v0.3.0-18-g441470e: 1563 - 1611 - 924  [0.494] 4098
Elo difference: -4.07 +/- 9.35
SPRT: llr -1.44, lbound -2.94, ubound 2.94
Finished match

Score of Little Wing v0.3.0-34-g9922948 vs Little Wing v0.3.0-18-g441470e: 948 - 1008 - 554  [0.488] 2510
Elo difference: -8.31 +/- 11.99
SPRT: llr -1.44, lbound -2.94, ubound 2.94
Finished match