generalize Align4::as_slice for any Sized type with appropriate alignment

[lifning]

also move the assertions to compile-time because we know what we need to know by then

[Lokathor]

This looks reasonable, but now that the math is getting more complex, could you add some various tests for this?

[lifning]

should i add a test harness to the project in the first place too? or just add another example

[Lokathor]

oh, frik, right, we don't have tests in this crate.

yeah, an example use that just makes sure all these methods get called once or something would be good.

[lifning]

it felt more right to do a unit test, so i added a test harness after all (#212)

[Lokathor]

Uh, i think there's a typo where a comma is missing, so the test doesn't parse