sumtypes for D

[rikki cattermole]

For all we know Walter just threw together some stuff on a whim, hence 
why he hasn't posted what he has yet. I did something recently too, 
another attempt at trying to fix ModuleInfo exportation on Windows which 
hasn't gone great.

But yeah ML is the feature set of future successful languages. The more 
analysis you bake into a language the better its going to be long term 
and ML features allow you to do this very effectively.

Memory safety wrt. data structures is much higher on my list of needs 
than sum types.