Introducing `for let` and `for reduce`

[l00ker]

@christhekeele

One idea that came to me yesterday was

for binding sum = 0, x <- [1, 2, 3] do
  {x * 2, sum + x}
end

I was thinking bind instead of binding because there is already binding/0/1, so:

for bind sum = 0, x <- [1, 2, 3] do
  {x * 2, sum + x}
end