This is part of updating the implementation to have a single number type.
I have to admit, doing this, I'm hesitant again. I would really like len
functions to return Int, or to clarify in the types that std.range
accepts only integers. But let's experiment, implement this, and get a
feel for it before I make a call.
It started to get annoying to have to define it myself every time, so
let's just add it properly now. This also resolves the longstanding
issue in the RCL pretty-printer that we have no good way to print the
empty set -- now we do!
