set __ghcjsArray when slicing array

[bitonic]

I was not able to test this properly, but intuitively the old copying operations are wrong since they lose the __ghcjsArray property.

This PR together with ghcjs/ghcjs#619 fixes ghcjs/ghcjs#463 .

[achirkin]

I was looking at this commit and found a piece of unrelated code nearby that I do not understand:
if(e === null) e = r; creates an infinitely recursive array if the second argument of the function is null, e.g. h$newArray(3, null).
Is this an intended behavior? :) I see this is used in NewArrayArrayOp primop, but that implies if one of the subarrays is updated, all others get updated too, because they all point to the same array.
e.g.
x = h$newArray(3, null); x[0][1] = 2; does quite crazy stuff :)

[luite]

@achirkin yes that's intentional. This is for arrays of arrays, where the initial element value is the array itself (from compiler/prelude/primops.txt.pp):

primop  NewArrayArrayOp "newArrayArray#" GenPrimOp
   Int# -> State# s -> (# State# s, MutableArrayArray# s #)
   {Create a new mutable array of arrays with the specified number of elements,
    in the specified state thread, with each element recursively referring to the
    newly created array.}
   with
   out_of_line = True
   has_side_effects = True