print more than one arg on println
add set method to Env
add Operative and PFn types

an operative is a "function" that receives it's arguments unevaluated
a PFn is a function that has as a body a list of plang values (that is,
a function that has plang as body instead of native python code)
only resolve the left value of a pair in LeftPairResolver
have functionality to expand a whole pair
if it's an Operative pass args unevaled, if a Fn eval before call
add def operative that defined a function and adds it to env

(def funname (*args) *body)