definitions in control structures

[robertwb]

Class and function definitions cannot be placed inside control structures.

[scoder]

For functions, this requires splitting the DefNode? up into the Def part and a separate assignment. Redefining functions must then be enabled, too.

For classes, this requires the use of head-allocated classes, which also serve various other helpful purposes, such as the truly module-local setup in Py3.

[scoder]

Make that "heap allocated classes" ...