Set theory objects in `org.cactoos.set`

See #1400, release log:

• 85f5d30 by @yegor256: Merge pull request #1779 from ...
• f7990c3 by @alimansour0002: Merge branch 'yegor256:master'...
• 23f2738 by @alimansour0002: feat: Add Diff class for set d...
• 08d52c6 by @rultor: Merge branch '__rultor'
• a3ea524 by @alimansour0002: This commit adds a test to ens...
• f74a2a3 by @renovate[bot]: Update dependency org.junit.ju...
• 4f1450f by @alimansour0002: ix Sliced iterator's end predi...
• 7d6f5a2 by @yegor256: Merge pull request #1776 from ...
• 01b21f1 by @yegor256: Merge branch 'master' of githu...
• a1623c5 by @yegor256: reuse
• 377a4d0 by @Alan-95-hub: use 2 spaces
• 06d5126 by @Alan-95-hub: small fix
• 291cd13 by @Alan-95-hub: convert to 4 spaces
• 3fffcda by @yegor256: Merge pull request #1775 from ...
• 6d37c24 by @Alan-95-hub: add iterable tests
• 19767b1 by @yegor256: Merge pull request #1774 from ...
• a499fe1 by @Alan-95-hub: add tests for all constructors
• af19633 by @yegor256: Merge pull request #1753 from ...
• 3024eaa by @yegor256: Merge pull request #1773 from ...
• 5c8a25b by @yegor256: Merge pull request #1772 from ...
• and 83 more...

Released by Rultor 2.0-SNAPSHOT, see build log