The new version of mon fixed the SeparatedBy combinator to not allow
trailing delimiters. This broke the Depend expr parser, because the
exprs are padded with whitespace. Using the new
SeparatedByWithTrailing combinator fixes this issue.
All atoms must either have a version with a version operator, or have
no version and no version operator. Putting these in the same Option
helps encode that into the type system.
