Acculate closure conversion

[KeenS]

Idea:
separate function-like, closure and function in type.
Then type driven closure conversion

val f: function-like = if xx then g:  function else h: closure

↓

val f: closure = if xx then force-closure(g):  closure else h: closure

,

val f: function-like = if xx then g:  function else h: function

↓

val f: function = if xx then g:  closure else h: function

[KeenS]

Space-efficient closure representations | ACM SIGPLAN Lisp Pointers

[KeenS]

http://mlton.org/References#CejtinEtAl00
https://github.com/MLton/mlton/blob/master/mlton/closure-convert/closure-convert.fun