Use subtree size for root move ordering

[FitzOReilly]

New move ordering at the root

• select PV move
• select moves that raised alpha during previous iteration
• order remaining moves by decreasing subtree size

[codecov[bot]]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Files	Coverage Δ
search/src/alpha_beta.rs	98.66% <100.00%> (+0.02%)	:arrow_up:
search/src/move_selector.rs	96.58% <100.00%> (+0.04%)	:arrow_up:
search/src/search_data.rs	99.45% <100.00%> (+0.01%)	:arrow_up:
search/src/move_candidates.rs	97.43% <96.66%> (-2.57%)	:arrow_down:

... and 1 file with indirect coverage changes

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