New function for adding to the address unsafely.

Would used an increment size based on mantissa width, but this is not
available in the type class, so just use 1 byte.
This function is currently used for adding small increments to the PC,
so does not need support for larger offsets.

CHERICap.bsv

@@ -124,6 +124,8 @@ typeclass CHERICap#(type t, numeric type ot, numeric type flg, numeric type n, n
   function Exact#(t) setAddr (t cap, Bit#(n) addr);
   // Set the address of the capability. Result assumed to be representable
   function t setAddrUnsafe (t cap, Bit#(n) addr);
+  // Add to the address of the capability. Result assumed to be representable
+  function t addAddrUnsafe (t cap, Bit#(8) inc);
 
   // Get the offset of the capability
   function Bit#(n) getOffset (t cap) = getAddr(cap) - getBase(cap);