RFC: make print polymorphic and fluent

[oxinabox]

Putting this out there for comments as to if it is a good idea. If it is then this PR still needs tests added.

• Making it call show on its input just seems more convient
• Making it return its input means you can (for debugging purposes) slip it inside some compouind expression
  • e.g. x |> f |>g can become x |> f |> print |> g

idk how useful this really is.

[dougalm]

An alternative would be to have three different functions, like Haskell:

def putString (s:String) : {State World} Unit = ...

def print (_:Show a) ?=> (x:a) : {State World} Unit =
  putString $ show x

def trace (s:String) : String = unsafeIO do
  putString s
  s

Notice that trace doesn't need the IO effect exposed, because it won't get DCEd as long as its result is used.