Less redundant

Remove spurious nil checks in set operations

Less redundant

• Repetitive set operations are all now generated using three straight-forward macros. The first of them was already existing; the remaining are now added.
• A few more tests!

Almost there!

• Fix resolution of methods defined directly in the module SparseBitSet. This caused errors in third-party code that used BitSet.
• Update tests accordingly.

Small refactoring release

popcount method has been refactored based on the notation in this Wikipedia page.

Improved performance release

• BitSet has been made a struct. This improves performance of creation of short-lived bitsets.
• Benchmarks have been refactored into their own directory.

Reasonably stable release

This release is reasonably correct and stable.

First feature-complete release

• No (de)serialisation support yet.