Replace tempdir with tempfile

[alexanderkjall]

as the tempdir crate has been superseded by tempfile

[JohnTitor]

So, tempfile breaks our MSRV on CI, however it's not much important (IMO) and alternatively, we could do cargo check only on MSRV CI. It doesn't affect user code in general but the downside is that we couldn't ensure the behavior (though I don't think it's a major issue).

[thomcc]

Yeah, I don't think test-only MSRV changes matter. It probably is worth checking that the crate compiles under MSRV for now (well, I don't feel that strongly here, but if we care about MSRV we should), but we don't need to run/build the tests.

[thomcc]

(OTOH I also don't think it's particularly important to update a test-only dep, especially if it causes complexity elsewhere. So your call)

[JohnTitor]

I'm going close in favor of #108 (comment), at least tempdir works fine for now. Let's revisit once we bump up MSRV, thanks for the PR anyway!