[System.Data] Delay column expression evaluation after all the columns have been...

[System.Data] Delay column expression evaluation after all the columns have been cloned when cloning a table. Fixes #666.

When cloning a table we loop over all the columns and clone them, but if a column
contains an expression with a reference to a later column, we'll throw an exception
unless we delay the expression compilation until all the columns have been added
to the new table clone.

So I've split out the expression compilation logic from the Expression property setter,
and now the column Clone method only sets the field. The Clone caller is responsible
for calling CompileExpression on the cloned column after adding all the cloned columns to
the cloned table.