Update files using slicing syntax [] to have the feature attribute.

Right now Travis CI fails because of, e.g.:

```
src/s_expressions.rs:260:37: 260:57 error: slicing syntax is experimental
src/s_expressions.rs:260 l.push(List(arena.alloc(list)[]));

src/s_expressions.rs:260:37: 260:57 note: add #![feature(slicing_syntax)] to the crate attributes to enable
src/s_expressions.rs:260 l.push(List(arena.alloc(list)[]));
```