Use syn::meta::parser to parse comma separated lists

[jawoznia]

          Nitpick: It's not a gamechanger, but let's consider a small update here. In many places in sylvia we parse list of comma separated lists with different content. The method described in https://docs.rs/syn/latest/syn/meta/fn.parser.html can make our life easier and probably increase readability of the parser code.

Originally posted by @kulikthebird in #452 (comment)