Enumerated Unions (sum types)

[Richard (Rikki) Andrew Cattermole]

This proposal follows Walter's design in requiring names for elements.

This does not match D's library implementations.

It does not match how it is used in the literature.

It is an improvement, from 8 issues down to 1 that I was able to 
determine, but unlike Walter's this cannot be fixed. It is core to the 
design.

For the other languages listed, Rust supports names being optional and 
SML supports the type being optional.