Add previous_definition

[klmr]

When overriding existing functions form base packages, use the following helper function to dispatch to existing functions without hard-coding their exact namespace:

previous_definition = function (name) {
    # Skip the parent environment of the calling frame.
    get(name, envir = parent.env(parent.env(parent.frame())))
}

This allows multiple redefinitions layered on top of each other (unlike e.g. devtools::help, which does this “wrong”). For an example of usage, see this Stack Overflow answer.