result decoder

[mulias]

• Improve how error messages are constructed when a decoder fails. I still don't love it, but it's a step in the right direction.
• Add decoders and utilities to help with managing imperfect data. The result decoder introduces a composable way to let parts of a decoder fail, without failing the full decoder. The Result.errors and Decoder.mapError utilities help work with the result decoder.
• Provide an example of how these tools can be used to validate an array of data, filtering out and logging failures.