Create resolve_to utility

[rafalp]

Python variables are usually named using the snake_case, whereas majority of JS written today uses the camelCase. Ariadne will need to provide a way to resolve one to another.

The idea is to create resolver_for function taking single name argument and returning resolver for specified attr/key name, enabling explicit mapping as such:

resolvers_map = {
    "User": {
        "lastVisitedOn": resolve_to("last_visited_on")
    }
}

Other advantage of having such utility would be arbitrary field names:

resolvers_map = {
    "User": {
        "lastVisitedOn": resolve_to("last_visit")
    }
}