Sanitiser doesn't handle unused variables properly

When there are variables passed into the sanitiser that don't actually get exercised in the code, the sanitiser just seemingly ignores them (it doesn't try mangle them, nor does it create redirect entries for them). This leads to lexing issues in Herd later on.

Ideally, the sanitiser should detect unused variables and remove them. I figure this'll probably have to be done by looking for variables that didn't redirect.