First Draft: opUnwrapIfTrue

[Walter Bright]

Thank you for taking the time to develop this.

This is a bit simpler and doesn't require language changes:
```d
struct Result(T)
{
     bool hasValue;
     T value;

     bool get(out T x)
     {
         if (hasValue)
         {
             x = value;
             return true;
         }
         return false;
     }
}

void bar(int);

void foo()
{
     Result!int r;
     int x;
     if (r.get(x)) { bar(x); }
}
```
It also can be embedded in an expression and other constructs:
```d
int x;
switch (r.get(x) ? x : x.init)
{
     ...
}
```

To channel Andrei, destroy!