Allow String keys to be set for error messages

[jwoertink]

avram/src/avram/operation_errors.cr

Line 19 in a604962

def add_error(key : Symbol, message : String) : Nil

I have a case where I get errors back from an API JSON. I want to pass those along in the error messages, but I can't just add_error(err["key"].as_s, err["val"].as_s), and there's no way to convert JSON to Symbol, or a String to a Symbol.

The big issue here is that it becomes a major breaking change to allow both Symbol and String. I'm not sure what the solution is here, but we should definitely make this more flexible.