[Ur] interfacing with ruby

[Adam Chlipala]

Marc Weber wrote:
> val context : string -> value
> The context function always returns an integer which represents the ctx
> pointer. Its used to identify a request within Ruby.
>
> I had to add the "string ->" to make it compile because
> "context: value" did not.
>    

I would write it as:
     val context : transaction value

It should really be marked as impure like this, because the value may 
change between contexts.  In other words, this isn't a standard, purely 
mathematical function, unlike all the functions you can write staying 
within core Ur.