[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.
Find a correct version of this [here](https://github.com/D-Programming-Language/druntime/blob/f888fbcff82f30c549a7cccd98191dedcc3101ec/src/core/internal/traits.d#L32).