reducing NUMERIC size for 9.1, take two

Here's a second version of the main patch, in which I have attempted
to respond to Tom's concerns/suggestions.

(There is still a small, side issue with numeric_maximum_size() which
I plan to fix, but this patch is the good stuff.)

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise Postgres Company