Rename predicate.WrapIn() to If()

[ccremer]

Summary

• Renames predicate.WrapIn() to predicate.If()
• Changes signature of predicate.If(...) so that the parameters are interchanged.

This change makes it easier to read a nested pipeline like

predicate.If(authenticated(), pipeline.NewStepFromFunc("query db", ...))

Checklist

• Categorize the PR by setting a good title and adding one of the labels:
  fix, enhancement, documentation, change, breaking, dependency
  as they show up in the changelog
• Update documentation.
• Update tests.
• Link this PR to related issues.