Improve OptimizedSolver, use pieces as hash key

[sugyan]

before: #2

after:

test bench_3x20_optimized           ... bench:     200,705 ns/iter (+/- 10,797)
test bench_3x20_optimized_unique    ... bench:     198,053 ns/iter (+/- 5,818)
test bench_4x15_optimized           ... bench:   4,885,066 ns/iter (+/- 859,894)
test bench_4x15_optimized_unique    ... bench:   4,581,341 ns/iter (+/- 1,451,962)
test bench_5x12_optimized           ... bench:  25,122,333 ns/iter (+/- 3,643,952)
test bench_5x12_optimized_unique    ... bench:  23,692,666 ns/iter (+/- 2,720,265)
test bench_6x10_optimized           ... bench:  61,437,370 ns/iter (+/- 4,026,847)
test bench_6x10_optimized_unique    ... bench:  59,592,208 ns/iter (+/- 5,773,675)
test bench_8x8_2x2_optimized        ... bench:  13,205,620 ns/iter (+/- 292,175)
test bench_8x8_2x2_optimized_unique ... bench:  13,134,995 ns/iter (+/- 311,008)

[sugyan]

test bench_3x20_optimized           ... bench:     187,026 ns/iter (+/- 7,095)
test bench_3x20_optimized_unique    ... bench:     187,995 ns/iter (+/- 10,232)
test bench_4x15_optimized           ... bench:   4,001,075 ns/iter (+/- 98,575)
test bench_4x15_optimized_unique    ... bench:   3,949,183 ns/iter (+/- 529,813)
test bench_5x12_optimized           ... bench:  22,330,458 ns/iter (+/- 617,846)
test bench_5x12_optimized_unique    ... bench:  21,626,566 ns/iter (+/- 1,001,477)
test bench_6x10_optimized           ... bench:  56,913,308 ns/iter (+/- 3,558,654)
test bench_6x10_optimized_unique    ... bench:  55,484,554 ns/iter (+/- 4,135,125)
test bench_8x8_2x2_optimized        ... bench:  12,824,579 ns/iter (+/- 672,285)
test bench_8x8_2x2_optimized_unique ... bench:  12,703,054 ns/iter (+/- 194,899)