clean up std.ArrayList

* 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.

std/debug/index.zig

@@ -88,6 +88,16 @@ pub fn assert(ok: bool) void {
     }
 }
 
+/// TODO: add `==` operator for `error_union == error_set`, and then
+/// remove this function
+pub fn assertError(value: var, expected_error: error) void {
+    if (value) {
+        @panic("expected error");
+    } else |actual_error| {
+        assert(actual_error == expected_error);
+    }
+}
+
 /// Call this function when you want to panic if the condition is not true.
 /// If `ok` is `false`, this function will panic in every release mode.
 pub fn assertOrPanic(ok: bool) void {