Remove unsafe and unused code

[daira]

fixes #1

[nuttycom]

This comment should be retained.

[daira]

It isn't actually true for the replacement code (which performs at least the same number of memory operations as naive swapping would). The only part that remains true is "This is not strictly equivalent to swapping", which is implied by the new comment.

[nuttycom]

Are there any unit tests and/or benchmarks that could be brought over from the original sources to verify the correctness of this transformation, and determine how it compares in terms of runtime cost? I'm 👎 on this without more extensive tests at least.

[daira]

Yes, I've added tests in #3. (Note that these don't exercise the specific nondeterministic behaviour we're trying to preserve, though, they're just the tests from the stdlib.)