This is an XY problem, but the Y is interesting in itself.
X: I want some examples of implementations of the fmt.Formatter interface to see how to effectively use it. *big.Int is one that I've found, but I'd like to find more. Searching for "Format" works fine for this, and it seems there are no others.
Y: Has someone written anything that can find all types that satisfy an arbitrarily complex, but valid interface?