multi-partition inserts

We should allow queries like this into a partitioned table:

CREATE TABLE (a STRING, b STRING, c STRING)
PARTITIONED BY (ds STRING, ts STRING);

INSERT OVERWRITE TABLE x PARTITION (ds = '2009-12-12')
SELECT a, b, c, ts FROM xxx;

Basically, allowing users to overwrite multiple partitions at a time.
The partition values specified in PARTITION part (if any) should be a prefix of the partition keys.
The rest of the partition keys goes to the end of the SELECT expression list.