powerset: implement an LRU cache

* spot/twaalgos/powerset.cc: Use an LRU cache to fix #302.
9 jobs for adl/302 in 93 minutes (queued for 1 second)
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passed arch-clang #3529

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passed arch-gcc-glibcxxdebug #3530

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failed debian-gcc-snapshot #3546

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passed debian-stable-gcc #3526

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failed debian-unstable-gcc-coverage #3545

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passed mingw-shared #3531

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passed mingw-static #3532

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failed debian-gcc-snapshot #3528

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failed debian-unstable-gcc-coverage #3527

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debian-unstable-gcc-coverage Build
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