[Oberon] Serious type loophole in type case statements and a possible fix

[Andreas Pirklbauer]

   > It feels like they are not in the spirit of Oberon…
   > all because of fear of missing out on performance?

For those who want to run some before/after performance comparisons - I’d be
interested in hearing about the results - here is one possible implementation of
the modified type case statement which eliminates all loopholes:

http://github.com/andreaspirklbauer/Oberon-type-case-statement-without-loopholes

(to see what has changed relative to PO2013, do a diff between the first 2 commits)

Implementation cost: ~15 lines   (relative to PO2013)