Change the unions of the Table.addColOffset field from characters to bytes.

Change the unions of the Table.addColOffset field from characters to bytes.
This makes the query that implements ALTER TABLE ADD COLUMN more complex and
slightly slower, but also makes CREATE TABLE statement parsing faster by
avoiding a call to sqlite3UtfCharLen().  Since, CREATE TABLE parsing is far
more common than ALTER TABLE, this is a net win for performance.

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