Functions combinations(), combinationsAsync() added

[Smoren]

• Add functions combinations(), combinationsAsync() to namespace combinatorics.
• Add method combinations() to Stream and AsyncStream.
• Add tests for functions combinations(), combinationsAsync().
• Add tests for combinations() method of Stream .
• Add tests for combinations() method of AsyncStream.
• Update docs in README.md for combinations() function.
• Update docs in REAMDE.md for method combinations() of Stream and AsyncStream.
• Update CHANGELOG.md.
• Update package.json with a new release version (2.2.0)

[coveralls]

Pull Request Test Coverage Report for Build 13062134463

Details

• 27 of 27 (100.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 13030752012:	0.0%
Covered Lines:	1460
Relevant Lines:	1460

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💛 - Coveralls