Allowing either `Formula`e or `String`s in these methods leads to errors
at runtime when one hasn't done `require "formula"` yet.
Let's tighten these up so that they only accept `Formula` arguments to
guarantee that `require "formula"` has been done before they are called.
For callers that need to pass a `String`, we update them to call a
version of these methods that accepts only `String`s.
See discussion at #20352.
a76c33f7bb