Add a lint task to the Makefile

This gives developers a convenient way to run the static analysis checks
(which are fast) without running all the tests (slow).

I often find myself wanting to do this as a quick check that nothing has
gone weird when doing an interactive rebase. That would look like
```
git rebase -i -x "make lint"
```
(`-x` is ‘e**x**ecute’, so runs after each commit)