Introducing deferred:do macro

[kiwanami]

I'd add monadic notation macro in next version v0.6.0.

Here is a sample code:

(let (aa bb)
  (deferred:do
   (aa <- (deferred:process-shell "ls /var | wc -l"))
   (bb <- (deferred:liftd length (deferred:process-shell "ls /lib")))
   (lambda (x) 
     (message "var: %s / lib: %s" aa bb))))

The definition is the same as edbi:seq at edbi.el. https://github.com/kiwanami/emacs-edbi/blob/master/edbi.el#L119-L144