Allow trailing comma in `concat!`

(And in other extensions implemented with `get_exprs_from_tts` function).
This loop should probably be restructured to look something like:

```rust
while p.token != token::EOF {
    es.push(cx.expand_expr(p.parse_expr()));
    if p.eat(&token::COMMA) { continue }
    if p.token != token::EOF { /* error about trailing expressions */ }
}
```

In general I don't think that the error `expected token: ","` should show up?