utils: gdb: fix pretty-printing of BigNum

I was accumulating the digits in reverse...

utils/gdb/abacus_pretty_printers.py

@@ -1,5 +1,12 @@
 import gdb.printing
 
+def digits_to_num(digits):
+    res = 0
+    for d in reversed(digits):
+        res *= 10
+        res += d
+    return res
+
 class BigNumPrinter(object):
     "Print a abacus::bignum::BigNum"
 
@@ -9,11 +16,11 @@ class BigNumPrinter(object):
     def to_string(self):
         digits = self.val['digits_']
         begin, end = digits['_M_impl']['_M_start'], digits['_M_impl']['_M_finish']
-        val = 0
+        val = []
         while begin != end:
-            val *= 10
-            val += begin.dereference()
+            val += [begin.dereference()]
             begin += 1
+        val = digits_to_num(val)
         val *= self.val['sign_']
         return str(val)