Implement Mixed::GetTable

[bmunkholm]

No description provided.

[kspangsege]

I don't think this is possible.

The worst problem is that we cannot keep track at compile time whether we have read/write or const access to the subtable.

The other problem is that we would have to handle reference counting when copying and destroying a mixed value which will incur a runtime overhead even when the mixed value does not contain a subtable pointer.