[new AA] implement core.internal.aa

This PR introduces new AA implementation. At the first stage, compiler will not know about `core.internal.aa.AssociativeArray` and will use only object.aaNew and object.aaLiteral methods.
Thus, this AA implenetation should replace rt.aaA implemetation.
When first stage will be finished, we will start work to endowing `core.internal.aa.AssociativeArray` with all the abilities of language's AA. For example:
```
int[string][string] table;
table["foo"]["bar"] = 3;

AssociativeArray!(string, AssociativeArray!(string, int)) table2;
table2["foo"]["bar"] = 3; //exeption, because "foo" doesn't exist in table2
```
However, now I dont's talk about second stage. I want to finish first stage, wait one release cycle (to catching all possible troubles), and start to work under second stage when the next D version (after first stage finish) will be released.
Same here, move these definitions close to opEquals. There are pretty daunting, so they should not make the first impression when looking at AssociativeArray.