Style.
Clarification comment. Still the reasoning ununderstood.
Clarification comment.
Half-implement the autoloading of classes within namespaces.

When there is a rootmap file describing: MyNamespace::MyClass and MyNamespace
is seen for the first time, we end up with a lookup failure for MyNamespace.
To avoid that we just need to store compiler's state declare the namespace as:
namespace MyNamespace{}

On a lookup failure of MyNamespace, ROOT has to signal cling that there is a
class in namespace with that name. We needed some extension of TCling's and
TClingCallbacks' interfaces.
Implement autoloading of entities in unknown namespaces.
Eg. MyNamespace::MyClass.
The implementation is far from ideal, but for the timeline we have it is good
enough. On a second iteration it could be improved a lot.
The namespace must be our second decl in the transaction.