Switch to using bitset2.

[Y-Less]

This doesn't make a difference at this stage, but is a precursor to some other changes intended to generally improve memory consumption. Most importantly bitset2 has one very important new function - find_first. It isn't quite as efficient as my method was (see Hacker's Delight, ISBN 9780201914658), but I've forked the original project so we can add internal improvements as we need.

[Y-Less]

OK, Linux apparently has exceptions disabled, while Windows doesn't.

[Y-Less]

Which is weird, because throw is used in plenty of other places.

[Y-Less]

Hopefully fixed that, but as I wrote this as a precursor to some more changes, that aren't really needed since #550 does a good enough job for now, we can leave this for a bit.

[CLAassistant]

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