Allow empty sep for separated_list but not sep and f empty

[Stargateur]

Fix #1288

This make more sense to consider both should be empty to be in infinite loop, and follow expected behavior of:

let (input, output) = separated_list0(opt(char(',')), parser)(input);

equivalent to manually do:

let (input, output) = parser(input)?;
let acc = vec![output];
let (input, output) = fold_many0(
  preceded(opt(char(',')), parser),
  || acc,
  |mut acc, output| { acc.push(output); acc },
)(input)

where fold_many0 fail only if the both seq and item parser are empty.

[coveralls]

Coverage decreased (-2.8%) to 81.163% when pulling eeecd7d on Stargateur:fix_list_sep into 4a2f9af on Geal:master.

[dalcde]

Why was this PR closed?