Backport Mini-language Overhaul To Release 12 Branch

[Adrian Crum]

This subject has been mentioned in other threads, but I'm making it a 
separate thread so everyone has a chance to comment.

I would like to backport the Mini-language overhaul in its entirety to 
the Release 12 branch. The overhaul contains some new features (<break>, 
<continue>, <trace> elements, and element validation), but most of the 
work was bug fixes (the model classes were not thread-safe, many bits of 
code did not work). So, the backport is not purely a bug fix. Separating 
the code so that only bug fixes are backported would be difficult - 
because the overhaul represents 6 MB of changes.

I have backported the overhaul to Release 12 on my local machine and 
everything works fine. If there are no objections in the next few days, 
I would like to commit those changes.

-Adrian