Use grammar-delta merging to build theory grammars

Test-case illustrates this: as per the comment in the file, the old
method for merging grammars results in B's grammar having two rules for
"=", making it ambiguous and (because of the trace) causing any
subsequent parse to fail with an ambiguous grammar error.

The new method correctly merges grammar deltas, resulting in a grammar
with just one rule for =.