* add `std.debug.assertError`
 * `std.ArrayList` update everything to follow `self` convention
 * rename `std.ArrayList.set` to `std.ArrayList.setOrError`
 * add `std.ArrayList.set` which asserts

Before 1.0.0 we might remove some of this API, because you can use
`toSlice()` for everything, but it's ok to add these functions as
an experiment before then.