`..` can't be chained

[rokke-git]

in the precedence and associativity table we claim that : and .. are identical precedence and left-associative. this is true for :, but a..b..c is an error.

the rest of the prec-colon operators ( … ⁝ ⋮ ⋱ ⋰ ⋯ .… .⁝ .⋮ .⋱ .⋰ .⋯) are the same.

found this when writing a helper function to make writing n-d list comprehensions nicer:
iter..x = Iterators.product((iter for _ in 1:x)...)
[foo(a,b,c) for (a,b,c) in 1:n..3]

[JeffBezanson]

I'm not sure why it's like this; probably related to the unusual a:b:c ternary syntax, but I agree we should add this.