ROP "describe" should use argument names from functions.py

[heapcrash]

Currently, when calling e.g. rop.system("/bin/sh") and doing rop.dump() we might print out the address of /bin/sh with e.g. arg0 as the "description" for that ROP slot.

It would be useful if, when doing rop.call, we look up the function in @zachriggle's functions.py (https://github.com/zachriggle/functions/blob/master/functions.py) and use the argument name from there.

Since this is a huge file, it should be lazily-loaded.

In this case, it has a line:

 'system': Function(type='int', derefcnt=0, name='system', args=[Argument(type='char', derefcnt=1, name='command')]),

So we would be able to use char* command instead of arg0.