Initial response to feedback from Norbert, Makarius on record patch
As Norbert recommended, the IsTuple.thy and istuple_support.ML files
have been integrated into Record.thy and record.ML. I haven't merged
the structures - record.ML now contains Record and IsTupleSupport.
Some of the cosmetic changes Makarius requested have been made,
including renaming variables with camel-case and run-together names
and removing the tab character from the Author: line. Constants are
defined with definition rather than constdefs. The split_ex_prf
inner function has been cleaned up.