Too much kw::Empty usage

[nnethercote]

kw::Empty is the name of the empty Symbol. It's used a lot. Most places it's used, it's hard to tell if it means "empty symbol" or "no symbol". I.e. it more or less reinvents the null pointer, something that Rust was supposed to get rid of.

I think a lot of the uses could be removed by using Option<Symbol> instead of Symbol, making the "no symbol" cases much clearer.

[nnethercote]

#137977 is the first step here. The backstory: I grepped for kw::Empty, looked at the very first occurrence in the output, worked out how to eliminate it, removing 17 kw::Empty occurrences, and I unintentionally fixed a crashtest in the process. A promising start.