Inline some type aliases in public API, part 2

Follow up of #906

The same as in #906 needs to be done, but for the following aliases as well:

- AnyCol,
- AnyColumnReference,
- AnyColumnGroupAccessor,
- StringCol,
- ColumnGroupReference

The changes should not include internal usages, and the declarations of the aliases themselves should not be removed. The main idea is to make code completion in the public API cleaner.

Start by reading issue #906 and tracing the public API declarations that use AnyCol, AnyColumnReference, AnyColumnGroupAccessor, StringCol, and ColumnGroupReference. Check which usages are public versus internal, then verify that only public signatures inline the aliases, the alias declarations remain, and code completion presents the cleaner API.

Written by the indexing model from the issue text.