Relax comma rules to allow optional extra end comma

[@FroMage] I’d like us to relax grammar rules concerning end commas and allow optional trailing commas:

class Foo<A,
                   B,
                   C,
       >(A a,
           B b,
           C c,
         ){
 value seq = {
  a,
  b,
  c,
 };
 Foo<
  Integer,
  String,
  Character,
 >(
  1,
  "a",
  `b`,
 );
} 

This is a particularly terrible example, but allowing extra trailing commas is generally useful, so why not just allow it everywhere commas can be used?

[Migrated from ceylon/ceylon-spec#381]

[xkr47]

I like it for diff’s sake when you keep appending stuff (to lists or hierarchial data structures) as things evolve. Less changed lines. Also when reordering items it helps when you don’t have to keep shuffling the commas.

Priority-wise not very high for my part though 😃

[CeylonMigrationBot]

[@chochos] I don’t know about everywhere but certainly for list-oriented stuff it is indeed useful:

value seq = {
    one,
    two,
    three,
}; //and you can add or remove items later without having to touch the existing ones
", ".join(one, two, three,); //same here
largest(a,b,); //this should be an error because largest only expects 2 params, it's not sequenced