Fixing RETVAL code.

-RETVAL was getting None value because greenlet_entry should be
 returning the value.

-Also, printing the return of switch with str() could cause an infinite
 loop case the value is a recursive structure as it is sometimes the case.
Debugger with descriptive bt frame information (Closes #51)

-Constructed `kernel_cmodule` (the kernel compiled module) associated to
 the instance of the kernel module.
-Added `module` field to compiled classes
-Added `thisContext` and `thisModule` keywords to the language
-Added `top_level_cfun` field to compiled functions

...and a few smaller things.
Debugger is too slow to start. Please, be patient.