Rustaceans at the border

What we do is we have a separate git history for the import that is subset from upstream (remove test suites, test data, docs, sources we don't care about, etc.) and commit that on the respective tracking branch. One can even transform the import to do things like sqlite's amalgamate to make a smaller set of sources. This then gets `-Xsubtree` merged into the main tree and is checked that:

- changes to this directory only come from such a merge
- the merge came from the "right" branch (tracked by its root commit)
- the merge did not modify the subtree in the merge commit (yay)

We also check that *all* such changes come in through this mechanism and we track our patches in a fork of upstream that we tag for each import for posterity.