Make SolidDatasets and Things atomic objects

This allowed for the removal of many brittle heuristics, allows
for better integration with the wider ecosystem, ensures that no
methods are present but intended not to be used, makes debugging
easier for developers, and is likely to improve performance (though
this wasn't measures). It also allows developers to obtain the data
in a way that integrates with the wider RDF/JS ecosystem.

Author: Vinnl

CHANGELOG.md

@@ -6,6 +6,22 @@ The following changes have been implemented but not released yet:
 
 ## [Unreleased]
 
+### Changed
+
+- `getUrl` now returns local URLs (e.g. #my-thing)
+- `getThing` now returns empty Things if they were added, rather than `null`.
+- The internal structure of SolidDatasets has been reworked. This shouldn't
+  affect you if you relied on just our public API's, but if you were
+  manipulating the data directly, you might need to update to use our public
+  API's. If something's not covered that you need, please [file an
+  issue](https://github.com/inrupt/solid-client-js/issues/new/choose).
+
+### New features
+
+- `getSolidDataset` now returns atomic objects, making it easier to integrate
+  with other tools in the JavaScript ecosystem, easier to debug, and easier to
+  serialise.
+
 The following sections document changes that have been released already:
 
 ## [1.7.0] - 2021-05-06