Leopard: "Letter of" block should convert input to string first

[PullJosh]

Identified here when analyzing KarateToast's first project. letter (n) of (110101010110111) was treating the input like a number and returning undefined.

This is sort of the fault of the compile-time check that turns variable & list values into numbers where appropriate, but I think the best solution is to always run input.toString()[n].