[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.
I remembered why I pass arguments by pointer.
In some places (like line 561) I need to cast argument from const to mutable. When I cast l-value, it becomes r-value: `e.key` is l-value, `cast(Key)&e.key` is r-value. Pointers helps avoid this trouble.