add `define`

[hamidb80]

  (1 .. 10) |> define( 
    a = it + 1
    b = a +2
  )
  .map(b)
  .toseq()

[hamidb80]

it's really hard to implement, I think saving into tuple next to the iterator variable is good solution

for example instead of above code, you can write:

  (1 .. 10) |> map((it, it+1, it+1+2))
  .map((it, a, b) => b)
  .toseq()