Introduce new structures for storing SQL statements.
This introduces SQLResult and AlterTableSQLResult, which are used to

return and store lists of SQL statements.
SQLResult has three fields: pre_sql, sql, and post_sql. pre_sql and

post_sql are used to store SQL statements that should be executed

before/after sql. These can be converted to a single list of SQL

statements with the to_sql() function.
AlterTableSQLResult extends this to also have an alter_table field that

contains a list of ALTER TABLE rules. When calling to_sql(), these are

turned into ALTER TABLE statements.
Where these structures come into play is with the next round of

optimizations. By knowing up-front what SQL statements must be executed

before/after a main set of statements, and which ALTER TABLE rules are

going to be executed, a post-processor will be able to batch operations

in an intelligent way. For example, joining all the results together to

drop all indexes before a batch of statements, combining ALTER TABLE

rules into a single ALTER TABLE statement, and then creating all new

indexes after they execute.
That will come in the next big change.

All unit tests pass for all supported database types.
The SQL result of an upgrade from RB 1.7.x to 2.0 remained unchanged,

since optimizations aren't added yet.