insert() can be faster

See #88, release log:

• 558a5e2 by @yegor256: Merge pull request #89 from ye...
• a85bd1d by @yegor256: new benchmark results
• fd59929 by @yegor256: #88 insert() tested and fixed
• afe29b4 by @yegor256: #88 Option instead of Pair
• 93398cf by @yegor256: #88 no remove() inside insert(...
• 56e4205 by @yegor256: #88 better testing
• 999df62 by @yegor256: Merge pull request #85 from ye...
• 06fc1e6 by @yegor256: Merge pull request #86 from ye...
• eb38008 by @yegor256: new benchmark results
• d7d1ded by @rultor: new version in README

Released by Rultor 1.74.7, see build log