stipulate `sep` parser must consume

[gdennie]

separated_list1 by multipspace0 sep failed #1691

the sep parser in the separated_list parser combinators cannot be "any" parser but must be a consuming parser, contrary to documentation. Ideally, we should have a trait to assert this constraint as well but better clarity in the documentation helps.

[Geal]

thank you, it's a good idea to be explicit about it