Issue 3553/2: When creating a hierarchy above Bottom, all intermediate levels are...

Issue 3553/2: When creating a hierarchy above Bottom, all intermediate levels are named "\new"

Previously, get_default_interpreter gave all intermediate levels the
same id as that of the new Bottom level.

However, since issue 3225, there can be more than one Bottom in one
hierarchy since a Bottom context (defined by not having a
\defaultchild) can still have children contexts (of any type it
\accepts even while it would not create them implicitly).

So it is a bad idea to give implicitly created contexts the same id as
any explicitly named context.