Exception declarations are now done entirely in EEL_xno.h

Macroified EEL_xno.h and turned the comments into actual
descriptions that can be acquired with eel_x_description()
or eelbil.exception_description().

The implementations are all macro based, generating code
from EEL_ALLEXCEPTIONS in EEL_xno.h, so exception codes,
names and descriptions can no loger go out of sync due to
human errors.