packed: move old vector helpers aside

We're going to replace it with a Vector trait that can be implemented
for multiple vector types. Similar to what was done with memchr[1].

Before doing that, we move the old stuff aside.

[1]: https://github.com/BurntSushi/memchr/blob/f6188dbbca2b529100852e4509d48d2d002a674a/src/vector.rs

src/packed/mod.rs

@@ -113,4 +113,4 @@ mod teddy;
 #[cfg(all(feature = "std", test))]
 mod tests;
 #[cfg(all(feature = "std", target_arch = "x86_64"))]
-mod vector;
+mod vectorold;