Reorganize contracts for better discoverability and clarity

[frangio]

We've identified that users are not always aware of all of the contracts that we offer, and that it can be difficult to find the things they want.

To improve this situation we want to reorganize the contracts so that they can be more easily discoverable, and that the directory structure is clearer and more useful to find a specific contract.

The specific of this reorganization will be discussed in the corresponding PR.

Since this is a breaking change due to files moving around, we've evaluated some alternatives to either make this change backwards compatible or to mitigate the pain it will cause to upgrade. One option was to include files with import statements to redirect the previous import paths to the new ones, but this would have the undesirable consequence of polluting the repository or the npm package with many (pseudo-)duplicate files that would hinder the discoverability improvements we set out to make. In the end we've decided to ship a small script that can be used when upgrading to adjust import statements to their new paths.