[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.
@MartinNowak I've added the test but I found strange dmd situation (dmd bug?).
If I do this struct as non-template, I've got a undefinded reference to Test5.init if I compiles some module with `-unittest` and import `core.internal.aa` in it. Of cource, druntime is builded as without unittest and doens't contain this symbols. (Thay placed inside `version(unittest)`). I don't know who refers to this symbols.