Add more precise types to important modules

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`