Add comments about the performance of some methods

[bobg]

This PR adds comments about the performance of some methods, as suggested in #19.

[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 patchlevel.

checking sliceIter
  old and new versions of github.com/bobg/go-generics/v3/iter.sliceIter[S ~[]T, T any] are not identical
    Patchlevel

[coveralls]

Pull Request Test Coverage Report for Build 10190862711

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 87.854%

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Totals
Change from base Build 10021976838:	0.0%
Covered Lines:	1056
Relevant Lines:	1202

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💛 - Coveralls