Add doublechecked::Roaring64Map + tests, fix issues found by it

[SLieve]

The implementation of both doublechecked::Roaring64Map and the tests are mostly copied from the 32-bit version. I ran clang-format on the newly added code in tests/cpp_random_unit.cpp. I also added a statement to print the random seed, to reproduce failures only reproduced by specific seeds.

Adding this random test surfaced a few issues:

1. Roaring64Map::isSubset did not work correctly when a Roaring value was empty. It would return false if the key for that value was not found in the other Roaring64Map. The correct behavior is to return true, just as empty Roarings return true for all isSubset calls.
2. Roaring64Map::flip did not check for a valid range.
3. convert_to_bitset_or_array_container would loop forever if run_end == uint16 max.

[lemire]

This looks very good to me.