Support paths on literal expressions (constants)

#589 adds a test that looks like:

```rust
/// cbindgen:enum-class=false
#[repr(C)]
pub enum E {
    V,
}
use E::*;

pub const C2: E = V;
```

It seems it should be doable to make `cbindgen:enum-class` not mandatory, if we support `E::V` as a proper literal.