Remove spurious `nil` checks in set operations

Update in-place operations and cardinality methods to use macros

* Add a few more tests.
* Increase version to `v0.5.0`.
* Update `README` to specify `shard.yml` as the installation method.

Fix path resolution for methods directly in module

* Increase version to 'v0.4.1'.

Increase version to 'v0.3.1'

Refactor benchmarks into a different directory

* Tag 'v0.3.0'

Increase version number to 'v0.1.0'

Add tests for all public methods

* Fix a few bugs as well.