* enforce a non-None parent in build_class

We also remove `add_local_node` to avoid redundancy. Instead we do the
   attachment to the parent scope in the constructor of `ClassDef`.

We append a node to the body of the frame when it is also the
   parent. If it's not a parent, then the node should belong to the
   "body" of the parent if it existed. An example is a definition
   within an "if", where the parent is the If node, but the frame is
   the whole module.

it's a part of the campaign to get rid of non-module roots

it's a part of the campaign to get rid of non-module roots