Fix for dropped columns in a partitioned table's default partition

 Fix for dropped columns in a partitioned table's default partition
 
 We forgot to map column numbers to/from the default partition for
 various operations, leading to valid cases failing with spurious
 errors, such as
 ERROR:  attribute N of type some_partition has been dropped
 
 It was also possible that the search for conflicting rows in the default
 partition when attaching another partition would fail to detect some.
 Secondarily, it was also possible that such a search should be skipped
 (because the constraint was implied) but wasn't.
 
 Fix all this by mapping column numbers when necessary.
 
 Reported by: Daniel Wilches
 Author: Amit Langote
 Discussion: https://postgr.es/m/15873-8c61945d6b3ef87c@postgresql.org