RFC: `..` as a right-associative cons operator

I've been spending some time writing functional programming libraries for Julia and I'm finding myself in want of a right-associative cons operator. I had a quick chat with @StefanKarpinski and he suggested `..`, which is currently unused.

Current parsing
```.jl
julia> :(x..y..rest)
ERROR: syntax: missing separator in tuple
```

Suggested parsing:
```.jl
julia> :(x..y..rest)
:(..(x, ..(y, rest)))
```

I don't think this could break existing code since `1..2..3` currently throws a syntax error, though it could lead to confusion as it's not completely obvious why it might be right-associative.

A quick note: the change in julia-parser.scm seems to do the right thing and tests are passing, but I'm basically making changes without fully understanding the parser.