r? @Amanieu

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Looks like I finally got this working! 🎉

Some context for the review:

• While int32x4_t([a, b, c, d]) appears to work fine, int32x4_t([x; 4]) apparently doesn't, so I ended up adding more internal splat methods that go through swizzles to ensure such things are proper splats -- lots of tests failed without that.
• A bunch of tests failed when it Debugged as int32x4_t([1, 2, 3, 4]) with square brackets there. While we're allowed to change debug like that, it wasn't clear that it was really better with square brackets, so I have the macro manually emitting an impl Debug, with a helper function to polymorphize just to the array type rather than monoing for every simd type.
• The separate impl Debug wanted stability attributes, of course, which led to more refactoring of the macro so that I could get just the stability part and not all the docs too.

Overall, I'm super-happy to see diffs like

-    pub struct v32i8(
-        pub(crate) i8, pub(crate) i8, pub(crate) i8, pub(crate) i8, pub(crate) i8, pub(crate) i8, pub(crate) i8, pub(crate) i8,
-        pub(crate) i8, pub(crate) i8, pub(crate) i8, pub(crate) i8, pub(crate) i8, pub(crate) i8, pub(crate) i8, pub(crate) i8,
-        pub(crate) i8, pub(crate) i8, pub(crate) i8, pub(crate) i8, pub(crate) i8, pub(crate) i8, pub(crate) i8, pub(crate) i8,
-        pub(crate) i8, pub(crate) i8, pub(crate) i8, pub(crate) i8, pub(crate) i8, pub(crate) i8, pub(crate) i8, pub(crate) i8,
-    );
+    pub struct v32i8(32 x pub(crate) i8);

because man was that too much copy-paste for my taste.