[Truffle] Pulled in String#sum from Rubinius.

Author: nirvdrum

spec/truffle/tags/core/string/sum_tags.txt

@@ -97,6 +97,19 @@ def slice!(one, two=undefined)
     result
   end
 
+  def sum(bits=16)
+    bits = Rubinius::Type.coerce_to bits, Fixnum, :to_int
+    i = -1
+    sum = 0
+
+    sum += @data[i] while (i += 1) < @num_bytes
+    if bits > 0
+      sum & ((1 << bits) - 1)
+    else
+      sum
+    end
+  end
+
   def to_c
     Complexifier.new(self).convert
   end