add closures to leema

[mdg]

Need to be able to make and return closures.
These will be also be used in asynchronous forks.

Still need to work out some of the syntax. Maybe something like:

func make_inc(): Int => Int ->
   fn(x) -> x + 1 --
--

or in the alternative model described in #16

func make_inc: Int -> Int
>>
   fn(x) >> x + 1 --
--

func make_inc_short: Int -> Int
>>
   fn(x) x + 1
--

[mdg]

pausing this for now while looking at adding currying in #20 which I hope will make this easier

[mdg]

back on this now b/c looks like it's easier to implement currying in terms of closures rather than the other way around.

[mdg]

done.
https://github.com/mdg/leema/blob/master/T/test_closures.lma