add slice method to Fiddle::MemoryView

[dsisnero]

# Fiddle::MemoryView#slice(start, length)
# Fiddle::MemoryView#slice(range)
mv = Fiddle::MemoryView.new(memory_object)

while mv
  puts mv[0]
  mv = mv.slice(1..-1) # returns a new MemoryView object with the same MemoryView#obj   # that is no copying
end