Condense ALTER TABLEs when possible.
When possible, multiple ALTER TABLE rules are now combined, resulting in

fewer table rebuilds. Adjacent ADD COLUMN, MODIFY COLUMN, and

DELETE COLUMN operations within a single batch of operations will be

condensed together.
When processing operations, AlterTableSQLResult will take the various

Alter Table operations and put them into batches of ALTER TABLE

statements. Typically, all operations in a batch will be turned into one

statement, but an Alter Table entry can set independent=True to ensure

it's part of its own ALTER TABLE, in the proper order.
generate_table_op_sql takes the operations for a model and combines

their SQL into an AlterTableSQLResult. If there are multiple adjacent

operations in a row that can be merged together (add_column,

change_column, delete_column), they'll be placed into the same

AlterTableSQLResult.
The result is a fairly compact list of ALTER TABLE statements that

trigger table rebuilds. It's still dependent on the ordering of

mutations, though.

Unit tests pass for all databases.
Checked SQL output for an RB 1.7.x to 2.0 upgrade. The number of

ALTER TABLE statements dropped by 3 (leaving 20 table-rebuilding

statements for 13 tables). Future changes should get this down to 13

for 13.