Merging back Diff refactor

Branch for diff refactoring
r11566@proteus (orig r1319):  ashberlin | 2007-11-27 13:34:56 +0000
MAss diff changes imported from Ash's local diff-refactor branch
r12366@proteus (orig r1320):  ashberlin | 2007-12-13 17:53:49 +0000
Some work on sanely normalizing fields
r12369@proteus (orig r1321):  ashberlin | 2007-12-16 22:24:00 +0000
Some SQL_TYPE mapping stuff
r12748@proteus (orig r1324):  ashberlin | 2007-12-20 16:54:56 +0000
Fix some more normalization problems
r12749@proteus (orig r1325):  ashberlin | 2007-12-20 17:23:44 +0000
Remove breakpoints
r12798@proteus (orig r1327):  ashberlin | 2007-12-21 10:02:30 +0000
Fix test
r13120@proteus (orig r1328):  ashberlin | 2008-01-10 13:55:19 +0000
Correct constraint names in preprocess for MySQL producer
r13152@proteus (orig r1329):  ashberlin | 2008-01-10 15:12:57 +0000
Move more normalization changes to preprocess_schema
r13244@proteus (orig r1331):  ashberlin | 2008-01-12 13:47:18 +0000
Fix after typo in merge
r13297@proteus (orig r1332):  ashberlin | 2008-01-15 23:34:54 +0000
Add renamed_from to tables.
r13309@proteus (orig r1333):  ashberlin | 2008-01-16 22:44:29 +0000
Start transactions in a portable manner
r13310@proteus (orig r1334):  ashberlin | 2008-01-16 23:47:59 +0000
Work round MySQL/InnoDB bug http://bugs.mysql.com/bug.php?id=13741
r13311@proteus (orig r1335):  ashberlin | 2008-01-16 23:50:00 +0000
Fix BEGIN in sqlite diff test
r13677@proteus (orig r1336):  ashberlin | 2008-01-21 16:22:51 +0000
Better tests (and fix bug) in batch alter with renamed tables + constraints