Add support for bulk statements.

[giant0791]

Add support for executing statements like:

# insertmany style:
stmt = insert(students).returning(students.c.name)

result = connection.execute(
    stmt,
    [
        {"name": "Galileo Galilei", "is_active": False, "grade": "A"},
        {"name": "Isaac Newton", "is_active": False, "grade": "B"},
        {"name": "Albert Einstein", "is_active": False, "grade": "C"},
    ]
)
# new_rows contains all the new pages just added, with the coulmn values for "name" only
new_rows = result.all()
assert len(rows) == 3

# inserted_primary_keys_rows return the inserted page ids as rows

# process bulk data
stmt = (
    update(students)
    .values(is_active = False)
    .where(students.c.start_on.before(date(1999,1,1))
    .returning(students.c.name)
)

result = connection.execute(stmt)
rows = result.all()

# rows contain all the students name that were updated
# sys cols are automatically available