Feature to get or add value to an associative array.

[Giles Bathgate]

Hi,

I wanted a way to lazily insert a value into an associative 
array, and I am proposing a new function called getOrAdd in  
https://github.com/dlang/druntime/pull/2162

The function provides a means to get a value corresponding to the 
key, but if the value doesn't exist it will evaluate the lazy 
argument to create a new value, add this to the associative array 
and then return it.

```
auto p = lookup.getOrAdd("giles", new Person);
p.eyeColor = Color.Brown;
//...
assert("giles" in lookup);
```

Corresponding documentation added in 
https://github.com/dlang/dlang.org/pull/2343

Implementation details:

Traditionally the above example could be done like so:
```
auto p = "giles" in lookup;
if (p is null) {
     p = new Person;
     lookup["giles"] = p;
}
p.eyeColor = Color.Brown;
//...
assert("giles" in lookup);
```

I find this later code clunky and it requires two hashes/lookups. 
The proposed implementation adds support directly to rt/aaA.d to 
avoid this. I should also point out that the allocation of a new 
Person in the example is trivial, but it might often be the case 
that the person instance is created by fetching a record from a 
db, or an Image loaded from disk, or a movie downloaded from the 
internet.