Better reciprocal division and modulus.

The previous reciprocal division branch on the prime that the sizeclass
was constructed from. All sizeclasses can be represented as
  2^n * {1,3,5,7}
This lead to a very small table, but some work to calculate the
appropriate shifts and multiplications to implement reciprocal division.

This commit uses a completely uniform representation for every
sizeclass using a lookup table.  Due to the precise ranges that we query
the modulus and rounding on, we can do this much more efficiently.

The func-release-rounding exhaustively tests all the queries we are
interested in.

src/test/func/memory/memory.cc

@@ -243,8 +243,11 @@ void test_external_pointer()
       void* p2 = pointer_offset(p1, offset);
       void* p3 = alloc->external_pointer(p2);
       void* p4 = alloc->external_pointer<End>(p2);
-      UNUSED(p3);
-      UNUSED(p4);
+      if (p1 != p3)
+      {
+        std::cout << "size: " << size << " offset: " << offset << " p1: " << p1
+                  << "  p3: " << p3 << std::endl;
+      }
       SNMALLOC_CHECK(p1 == p3);
       SNMALLOC_CHECK((size_t)p4 == (size_t)p1 + size - 1);
     }