[Oberon] Forward declarations of procedures in Oberon-07 on PO2013

[Andreas Pirklbauer]

A PO2013 implementation of forward declarations of procedures  (which have
been removed from the language in the Oberon-07 revision) is at:

http://github.com/andreaspirklbauer/Oberon-forward-declarations-of-procedures/

This should make it possible to directly compare (the run-time performance)
of nested procedures vs. procedure variables, e.g. in the compiler itself.

I have more or less followed the overall approach of the Ceres implementation,
but have made a number of different implementation decisions, in order to make
calls to such procedures optimally efficient (only a single BL instruction is issued
vs an external procedure call on Ceres)

-ap