Replace Term with a typealias of AnnotatedTerm<()>

[robrix]

Term is a special case of AnnotatedTerm<Annotation> where Annotation effectively carries no information, e.g. Annotation = ().

We could further rename AnnotatedTerm to Term, but that would require us to update all extant references to Term to instead name Term<()>.

[robrix]

1. We would probably have to say Unit instead of () where Unit is an empty struct conforming to Equatable.
2. We’d lose Equatable on Term, which might be a pain.