std.mem.asBytes: fix footgun when passing non-single pointer

I was just bitten by this footgun, where I actually wanted
`sliceAsBytes` but unintentionally used `asBytes`, which in practice
ignored all but the first element. Just add a comptime assertion to
trigger a compile error in this case.

lib/std/mem.zig

@@ -3965,7 +3965,9 @@ fn CopyPtrAttrs(
 }
 
 fn AsBytesReturnType(comptime P: type) type {
-    const size = @sizeOf(std.meta.Child(P));
+    const pointer = @typeInfo(P).pointer;
+    assert(pointer.size == .One);
+    const size = @sizeOf(pointer.child);
     return CopyPtrAttrs(P, .One, [size]u8);
 }