CheriBSD/CHERI support

This adds a CHERI AAL and expands the FreeBSD PAL to cover CHERI.  It updates a
comment in ds/address.h now that there is an example architecture that
differentiates uintptr_t and address_t.

src/ds/address.h

@@ -8,11 +8,8 @@
 namespace snmalloc
 {
   /**
-   * The type used for an address.  Currently, all addresses are assumed to be
-   * provenance-carrying values and so it is possible to cast back from the
-   * result of arithmetic on an address_t.  Eventually, this will want to be
-   * separated into two types, one for raw addresses and one for addresses that
-   * can be cast back to pointers.
+   * The type used for an address.  On CHERI, this is not a provenance-carrying
+   * value and so cannot be converted back to a pointer.
    */
   using address_t = Aal::address_t;