fix(memmove): move all bytes

Previously, when trying to move a region "backwards" with a length that wasn't a multiple of the word size, we'd move the first bytes twice (first byte-by-byte, then as part of the last word), and we'd fail to move `m` bytes from the very end.

Now, when trying to move 15 bytes to `add0` we will write to `add[2,1,0]` (or `add[0,1,2]`), then bump the pointer to `add3` where we'll write `add[f-c, b-8, 7-3]` (or `addr[3-7, 8-b, c-f]`)  a word at a time.

Demonstrating, once again, that it's better to run some examples when writing this kind of code: by hand, if necessary. I'm not sure how I convinced myself the pointer shouldn't advance when we're copying backwards, but there it was.

Author: sethp

src/lib.rs

@@ -123,19 +123,16 @@ mod mem {
         // If src is less than dst, just copy from the end.
         // If src is greater than dst, just copy from the beginning."
         // — https://stackoverflow.com/a/3572519/151464
-        let last;
         for i in if src < dest as *const u8 {
-            last = 0;
             Idx::Backward(m)
         } else {
-            last = m;
             Idx::Forward(0, m)
         } {
             *dest.add(i) = *src.add(i);
         }
 
-        let dest = dest.add(last).cast::<usize>();
-        let src = src.add(last).cast::<usize>();
+        let dest = dest.add(m).cast::<usize>();
+        let src = src.add(m).cast::<usize>();
 
         for i in if src < dest as *const usize {
             Idx::Backward(n)