Add tree deconstruction helper

[aantron]

This would convert DOM-like data structures into signal streams (the dual of Markup.tree):

val from_tree :
  ('a ->
    [ `Element of string * (name * string) * 'a list
    | `Text of string ]) ->
  'a ->
    (sync, signal) stream

with usage such as

type my_tree = Element of name * my_tree list | Text of string

dom
|> from_tree (function
  | Element (name, children) -> `Element (name, [], children)
  | Text s -> `Text s)
|> write_xml
|> to_string

Perhaps the result type of the function argument can support comments and others nodes besides text and elements.