Add support for finding constants with duplicated values.

[kalexmills]

Adds a flag for finding and reporting on duplicate constants, along with tests.
Also adds backwards-compatible fields to the Issue API for eventual use in golangci-lint.

I went ahead and set this new flag in the benchmarks, since it will result in additional allocations when set.

I'm open to making any changes requested by maintainers. Just let me know.

There are a few extensions I can imagine, but not all of them are quite as easy to implement.

• Limiting search within the same file, within the same package, or across packages. Currently this feature searches only across packages.
• Ignoring unexported constants when checking for duplicates.
• Adding support for constant expressions.

Closes #29.

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[jgautheron]

Hey @kalexmills thanks for the contribution, lgtm!