Remove high-level aggregation functions

Now that the nested-star-aggregator handles high-level aggregation, the relevant functions in this library can be removed to reduce the library size.

`serde_json` can also be removed once this is done.

Start by locating the high-level aggregation functions in this Rust library and the serde_json dependency, then inspect the nested-star-aggregator entry point to understand what now handles that work. Done means the redundant functions and serde_json are removed and the project still builds and passes its available checks.

Written by the indexing model from the issue text.