Refactor memcpy to allow different versions. (#472)

Expose a memcpy.h that contains all of the bits of memcpy and clean up
the bounds checks header so that versions with both read and write
checks can coexist.

src/test/perf/memcpy/memcpy.cc

@@ -1,11 +1,11 @@
+#include "mem/memcpy.h"
+
 #include <test/measuretime.h>
 #include <test/opt.h>
-
-#define SNMALLOC_NAME_MANGLE(a) our_##a
-#include "override/memcpy.cc"
-
 #include <vector>
 
+using namespace snmalloc;
+
 struct Shape
 {
   void* object;