Ad-hoc enum types

A Rust tuple could be considered as an ad-hoc struct for which the programmers do not need to name the type nor the fields. As an analogy, an ad-hoc enum is implicitly defined by its variants.

Unfortunately Rust does not support ad-hoc enums. This library uses Enum!() macros for simulation. For instance, the definition of Enum!(A,B,C) is as follows:

#![allow(unused)]
fn main() {
enum Enum3<A,B,C> {
    _0( A ),
    _1( B ),
    _2( C ),
}
}

The Enum!() macro denotes a series of generic enums named Enum0, Enum1, Enum2, .. which composed of 0,1,2.. variants. These enums should be defined beforehand, either predefined in this library, or defined by the library users.