First Draft: Callback For Matching Type

[Richard Andrew Cattermole (Rikki)]

This proposal is a subset of matching capabilities to allow for 
tagged unions to safely access with language support its values 
and handle each tag.

Some minor things have been changed from the ideas thread, I have 
changed the match block to be a declaration block to allow for 
``static foreach`` and other conditional compilation language 
features. So it is now using semicolon instead of colon.

```d
alias MTU = MyTaggedUnion!(int, float, string);

MTU mtu = MTU(1.5);

mtu.match {
	(float v) => writeln("a float! ", v);
	v => writeln("catch all! ", v);
};
```

Ideas thread: 
https://forum.dlang.org/post/chzxzjiwsxmvnkthbdyy@forum.dlang.org

Latest: 
https://gist.github.com/rikkimax/79cbe199618b3f99104f7df2fc2a9681

Permanent: 
https://gist.github.com/rikkimax/79cbe199618b3f99104f7df2fc2a9681/95ae646da1ebb079a522b0c993e3408e5a1c0d78