BitSet64: Fix iterator incrementation

Use 1 of the same type as the stored value when shifting left. This
prevents undefined behavior caused by shifting an int more than 31 bits.

Previously iterator incrementation could either hang or prematurely
report it had reached the end of the bitset.

Gets rid of the need to remember to initialize them in the constructor,
except when overriding the default initializer.

This was able to be constexpr since C++14.

seems to have been solved 5 years ago by
b707e199c2878d173c5d30f0a24db93d7de1d921

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