This prevents infinite recursion, similar to the fixes to the formatter
in #13.  In addition to detecting cycles, we also check that the two
values contain the same cyclic structure.

github.com/rogpeppe/go-internal factors out an opinionated selection of
internal packages and functionality from the Go standard library. One
such package is fmtsort:

    Package fmtsort provides a general stable ordering mechanism for
    maps, on behalf of the fmt and text/template packages. It is not
    guaranteed to be efficient and works only for types that are valid
    map keys.

Use this package to ensure the Formatter output for maps is stable.

Fixes #47

Print values using the `GoString()` method (from `fmt.GoStringer()`) if implemented.

This new version switches from obsolete github.com/kr/pty to
github.com/creak/pty, which has improved platform support.

This updates github.com/rogpeppe/go-internal from 1.6.1 to 1.8.0
and the github action 'setup-go' from v1 to v2.

It also adds a config file for dependabot to automatically check
this repo for new dependency versions weekly.

Bumps [actions/checkout](https://github.com/actions/checkout) from 2 to 3.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v2...v3)

---
updated-dependencies:
- dependency-name: actions/checkout
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

This handles a few cases (similar to how fmt %#v does):

- A GoString method on a value receiver, called with a nil pointer
- A GoString method on a pointer receiver that doesn't check for nil
- A GoString method that panics in some other way

Because Go 1.17 added a method Time.GoString with value receiver, this
broke structs that had *time.Time fields with nil values (which is
common!).

Also added a bunch of tests for these cases.

Fixes #77

Co-authored-by: Jordan Barrett <jordan.barrett@canonical.com>

This change bumps github.com/rogpeppe/go-internal from version 1.8.0 to 1.9.0.

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>