This is a start on using `test.check` to write tests for the vector instructions. We currently have tests for:
* `vector/_emptyvector`
* `vector/_indexof`
* `vector/_subvec`
There are _lots_ of other functions still to be tested.
This did reveal errors in `vector/_subvec`, which will be addressed in the next commit.
We used macros to make it easy to generate tests for each of the four vector types; this should be extensible to additional vector types in the future if needed.
