generate assoc. consts in enums representing protobuf enum aliases, resolves #792

[ousado]

This PR adds associated constants to enum impls representing enum aliases in protocol buffer enums as shown below. The aliases can be referenced by prost when used as default values, causing compilation to fail, see #792.

#[derive(PartialEq, Eq)]
pub enum X {
    A,
    B,
}

impl X {
    pub const A1: X = X::A;
}