Add some checks on the number of variables per type

Add some checks on the number of variables per type

We want to ensure no more than, say, 8 constants are added
to a program. Adding more will violate pervasive assumptions
in the code, and may lead to various buffer overflows. By
trapping these add creation time, we prevent these issues
without cluttering the code with range checks. The user is
assumed non malicious here.

Add a test to check we can add up to and including the limit
for a type, but no more.