Support paths on literal expressions (constants)

[emilio]

#589 adds a test that looks like:

/// 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.