New concept Mixin Methods

[Richard (Rikki) Andrew Cattermole]

A long time ago I considered something similar, its a very neat idea.

Where its needed most is at the module level, for any imports get it 
mixed in. This covers pretty much anything rather than being specific to 
classes.

But there is always improving your build manager... Its been on my todo 
list for a while now (quite low), to refactor dub's generated file which 
lists all modules so that it is available for more than just unittest 
builds.