Add slices.NonNil

[bobg]

Add slices.NonNil, for converting nil slices to non-nil empty slices.

Closes #10.

[github-actions]

Modver result

This report was generated by Modver,
a Go package and command that helps you obey semantic versioning rules in your Go module.

This PR requires (at least) an increase in your module's minor version number.

no object NonNil in old version of package github.com/bobg/go-generics/v3/slices
  Minor

[coveralls]

Pull Request Test Coverage Report for Build 9142835111

Details

• 5 of 5 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.2%) to 88.186%

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Totals
Change from base Build 7845733902:	0.2%
Covered Lines:	1045
Relevant Lines:	1185

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💛 - Coveralls