v0.5.6: `k_most_common_items` and more relaxed trait bounds

What's Changed

• Relax more trait bounds by @clint-white in #22
• Improve concision by @clint-white in #23
• Fix minor issues in documentation by @clint-white in #24
• Add method to return the k most common items and speed up most_common_*() methods by @clint-white in #25

Full Changelog: v0.5.5...v0.5.6