The new ANTLRv4 parser generates left-deep nested Boolean AND/OR
expressions, which need to be flattened where appropriate.  The
BooleanOperator.simplify() method takes care of this operation.

Introducing this flattening/simplification everywhere all at once seemed
to be inadvisable, so currently it is only applied in the query AST when
schemas are computed.  Additionally, only WHERE predicates, HAVING
predicates, and ON clauses are simplified/flattened.  (NATURAL JOIN and
USING join predicates are generated programmatically, not by the parser,
and are already flattened.)

The design document and information document are included under the doc
directory now.

Added a lot of documentation, particularly to the Schema class.

Made a few changes to the ArithmeticOperator and BooleanOperator
classes' simplify() methods so that we can do some basic
simplifications.

Added the StatisticsUpdater so that it's easier for students to modify
statistics based on selection predicates.

Fixed a bug in SelectClause where ORDER BY clauses were using the
FROM-clause schema rather than the SELECT-clause schema.

the `SelectClause.computeSchema` method aggregates a `resultSchema`
member by appending all of the ColumnInfos for each of the selectValues
in the statement.

However, when efforts were made to make Schema less "mutable", I think
a change was overlooked that causes all but the last selectValue's
ColumnInfos to remain in the resultSchema.

I believe this was left undetected because the resultSchema of the
SelectClause isn't really used in many places, except for nested
subqueries. This problem was found by a student, that the first out of
two columns in a nested subquery would "disappear" from the schema.