&[Trait] should probably not be allowed

This compiles:

``` rust
trait Reader {}
fn perform(readers: &[Reader]) -> usize { 0 }
```

and this complains that there is no `len()`:

``` rust
trait Reader {}
fn perform(readers: &[Reader]) -> usize { readers.len() }
```

It seems to me like both should fail to compile with an error about sticking DSTs in a slice.
