Add more precise types to important modules

[ilevkivskyi]

These modules require special attention:

• [ ] engine/base.pyi
• [ ] engine/result.pyi
• [ ] util/_collections.pyi
• [x] sql/base.pyi
• [x] sql/compiler.pyi
• [x] sql/ddl.pyi (?)
• [ ] sql/dml.pyi
• [x] sql/elements.pyi
• [x] sql/functions.pyi
• [ ] sql/operators.pyi (?)
• [x] sql/schema.pyi
• [ ] sql/selectable.pyi
• [ ] sql/sqltypes.pyi
• [ ] ext/instrumentation.pyi
• [ ] ext/horizontal_shard.pyi
• [ ] ext/declarative/api.pyi
• [ ] orm/__init__.pyi
• [ ] orm/attributes.pyi
• [ ] orm/interfaces.pyi
• [ ] orm/query.pyi
• [ ] orm/session.pyi