Compile splats in arrays and function calls to ES2015 splats (#4353)

Rather than compiling splats to arrays built using `Array#concat`, splats
are now compiled directly to ES2015 splats, e.g.

    f foo, arguments..., bar

    [ foo, arguments..., bar ]

Which used to be compiled to:

    f.apply(null, [foo].concat(slice.call(arguments), [bar]));

    [foo].concat(slice.call(arguments), [bar]);

Is now compiled to:

    f(foo, ...arguments, bar);

    [ foo, ...arguments, bar ];