Add decodeToml function to type class?

For some instances of DecodeTOML, it might be more ergonomic to define in terms of `Value -> DecodeM a`, even while the `Decoder` interface is good for the common case.

We could allow implementing either and define them in terms of each other

```hs
tomlDecoder = makeDecoder decodeToml
decodeToml = runDecoder tomlDecoder
```

Start by tracing the DecodeTOML type class and the Decoder, makeDecoder, and runDecoder interfaces named in the issue. Determine how supporting either implementation direction would affect the public API, then verify that both forms remain equivalent and are covered by the existing test suite; no test or source file is named in the issue.

Written by the indexing model from the issue text.