Refactor: Narrow the generic trait bound

See #259, release log:

• 1e5fa54 by @yegor256: Merge pull request #262 from y...
• 7c962d0 by @rultor: new benchmark results
• dbbb573 by @owtotwo: test: add unit test for retain...
• c4da034 by @owtotwo: fix: correct the retain() ap...
• 79463ee by @owtotwo: fix: correct the doc test.
• 1a42c5c by @owtotwo: Merge branch 'yegor256:master'...
• e905a86 by @yegor256: Merge pull request #261 from o...
• a957135 by @owtotwo: chore: remove blank lines in u...
• f08fa51 by @owtotwo: chore: remove blank lines in m...
• d2969cd by @owtotwo: refactor: use iterator's flatt...
• cb368d3 by @owtotwo: feat: add new api `get_disjoin...
• 26cbea4 by @owtotwo: Merge branch 'yegor256:master'...
• 858e9bd by @yegor256: Merge pull request #253 from y...
• 016ddb7 by @yegor256: Merge pull request #254 from y...
• ad38b3a by @owtotwo: refactor: remove all the not n...
• 91a8eca by @rultor: new benchmark results
• ff94037 by @rultor: new version in README
• 0e054a7 by @renovate[bot]: chore(deps): update rust crate...

Released by Rultor 2.0-SNAPSHOT, see build log