Detailed unmarshalling error information

[oioio]

Hi there.

I want to add more detailed information to unmarshalling errors, such as the struct instance and the name of the field where the error occurred. Would you be interested in such a PR?

How would be better to implement this?

1. Pass additional info in Lexer methods, like out.Value = in.String(out, "Value")
2. Add properties with current handling instance and field, to Lexer and set them before literals parsing, like

in.SetCurrent(out, "Value")
out.Value = in.String()

3. Return error from parsing methods and wrap them with more detailed error, like

out.Value, err := string(in.String())
in.AddParseError(err, out, "Value")

4. Something else?

I would prefer the third one.