[new AA] implement aaLiteral and aaInit

This functions will be used to constructing AA values: literals and nulls. When this PR will be merged, I will create PR for dmd, which uses this functions, and when dmd PR will be merged, I'll create PR, which rewrite this functions to use new AA implementation.
This is `void` slice, not `void*`. And yes, Key.sizeof can be not equals with void.sizeof. However `_d_assocarrayliteralTX` `void[]` takes slice with length == keys count. See `rt/aaA.d:562`:
```
Impl* _d_assocarrayliteralTX(const TypeInfo_AssociativeArray ti, void[] keys, void[] values)
{
    ...
    const length = keys.length;
    ...
    assert(length == values.length);
    ...
    for (size_t j = 0; j < length; j++)
    {   auto pkey = keys.ptr + j * keysize;
        auto pvalue = values.ptr + j * valuesize;
        ...
    }
    ...
}
```