Increase test coverage

[bobg]

This PR increases test coverage, and fixes one or two bugs that the new tests revealed.

It also removes the field Checker.Interfaces. It was used to control whether the checker looked for parameters whose types were interfaces but could be narrowed (such as io.ReadCloser -> io.Reader). It's now assumed that behavior is always wanted.

It also expands the Readme some.

[github-actions]

Pull Request Test Coverage Report for Build 5495962276

• 3 of 10 (30.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+14.0%) to 72.774%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
decouple.go	3	10	30.0%

Totals
Change from base Build 4669415880:	14.0%
Covered Lines:	572
Relevant Lines:	786

---

💛 - Coveralls

[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 an increase in your module’s major version number.
If the new major version number is 2 or greater,
you must also add or update the version suffix
on the module path defined in your go.mod file.
See the Go Modules Reference for more info.

checking NewCheckerFromPackages
  in results of func(pkgs []*golang.org/x/tools/go/packages.Package) github.com/bobg/decouple.Checker
    old struct field Interfaces was removed from github.com/bobg/decouple.Checker
      Major