Enable ROP chain dumping

Enable ROP chain dumping.  Here's an example.

```py
r.call('mmap', [0xBAD0F00D, 0x1000, READ|WRITE|EXEC, PRIVATE|ANON, -1, 0])
r.call('recv', [4, 0xBAD0F00D, 0x1000, 0])
r.call(0xBAD0F00D)

r.dump()
```


```
00000000: 55678ac0 mmap
00000004: 08048729 stackfix 11 slots
00000008: bad0f00d 
0000000c: 00001000 
00000010: 00000007 
00000014: 00000022 
00000018: ffffffff 
0000001c: 00000000 
00000020: deadbeef 
00000024: deadbeef 
00000028: deadbeef 
0000002c: deadbeef 
00000030: deadbeef 
00000034: 5567edf0 recv
00000038: bad0f00d 
0000003c: 00000004 
00000040: bad0f00d 
00000044: 00001000 
00000048: 00000000 
```