For the first time ever: names are handled quickly and correctly:
- Added e.defining_context and e.referencing_context fields.
- All ast. Name nodes now have an stc_e field.
- p1.lookup becomes just node.stc_e.defining_context: a *huge* speedup.
- Eliminated to-be-resolved lists and resulting complications:
- p0.define_name and p0.define_global_name now just call p0.add_name.

Simplified resolve_names:
- It now sets e.defining_context to relocate names.
- Builtin names could be redefined. No warning is given at present.

Fixed bug: p0.Lambda must visit args.