First Draft: opUnwrapIfTrue

[Richard Andrew Cattermole (Rikki)]

The DIP: 
https://gist.github.com/rikkimax/21242118e3bc1bf5f28024c2cdc33557

A new operator overload to allow if statements to automatically 
unwrap result types and ensure that the check is called prior to, 
similar to foreach statements support of ranges.

This solves a prolific issue that I have had to deal with result 
types and has been exhibited by ``Nullable`` in PhobosV2.

Full example:

```d
import core.attribute : mustuse;

@mustuse
struct Result(Type) {
     private {
         Type value;
         bool haveValue;
     }

     this(Type value) {
         this.value = value;
         this.haveValue = true;
     }

     bool opCast(T:bool)() => haveValue;

     Type opUnwrapIfTrue() {
         assert(haveValue);
         return value;
     }
}

Result!int result = Result!int(99);

if (int value = result) {
     // got a value!
     assert(value == 99);
} else {
     // oh noes an error or default init state
}
```