Macro cex::crate_error

macro_rules! crate_error {
    ( $( #[$attrs:meta] )* $vis:vis enum $ty:ident{ $($error:ident,)* }) => { ... };
}

Help to define a summarizing error type of the crate. The crate error should not appear in cex function’s signature, but for downstream users of cex functions, who do not want to adopt checked exceptions.