[Oberon] Importing string constants in PO

[Andreas Pirklbauer]

    > Peter Matthias PeterMatthias at web.de Tue Feb 20 21:01:06 CET 2018
    > > > Importing string constants does not work in PO.
    > > > Strings from same module are shown.
    >
    > > The fix would be to make string constants aware
    > > of modules. This require rewriting the compiler.
    >
    > Thanks for clarification. Although I think ORG.Mod should be updated to 
    > give an error when importing string constants, like I will do in my ORGx.Mod
    > Peter

Ok. So I couldn’t resist... although it’s easiest to simply disallow exporting
and re-importing string constants, I did the implementation:

http://github.com/andreaspirklbauer/Oberon-importing-string-constants

Implementation cost: ~5 lines.

PS: I have seen several different, more or less complex, ways to do this
over the years. Some store the string itself in the symbol file, others the
string buffer position (strx). I decided to store the string's export number
(exno) and treat string constant like exported variables..

-ap