Allow trailing punctuation in macros

[ModProg]

When splitting e.g. fields over multiple lines it is quite common to add a
trailing comma in the last line in rust code.

This pr makes it so that this is allowed now:

vector![
    some_longer_expression() + something,
    some_longer_expression() + something_else,
    some_longer_expression() + another_thing,
    some_longer_expression() + 42,
];

I also added a test verifying that this was already the case with matrix!.

[ModProg]

on another note, what was the rational to use a proc macro, AFAICT these macros look very doable in macro_rules, or was it just for ease of writing them (e.g. allowing easy reusing of code).

Edit: well they would require quite a bit of redundant code as declarative macros.

[sebcrozet]

Thanks! Yeah, using proc-macro makes the code simpler overall.