Round negative signed integer towards zero in `iN::midpoint`
This PR changes the implementation of `iN::midpoint` (the signed variants) to round negative signed integers **towards zero** *instead* of negative infinity as is currently the case.
This is done so that the obvious expectations[^1] of `midpoint(a, b) == midpoint(b, a)` and `midpoint(-a, -b) == -midpoint(a, b)` are true, which makes the even more obvious implementation `(a + b) / 2` always true.
The unsigned variants `uN::midpoint` (which are being [FCP-ed](https://github.com/rust-lang/rust/pull/131784#issuecomment-2417188117)) already rounds towards zero, so there is no consistency issue.
cc `@scottmcm`
r? `@dtolnay`
[^1]: https://github.com/rust-lang/rust/issues/110840#issuecomment-2336753931
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