Contract constraints should allow access to BLOCKNUMBER

[anton-trunov]

Some contracts depend on the notion of time which we model using BLOCKNUMBER variable. E.g. if a contract defines some kind of end-of-life of the contract in its parameters, then it would be unfortunate to deploy a newborn contract which is effectively dead. Contract constraints in this case would looks look something like this

contract Mortal (end_of_life : BNum)
with
  let current_time = BLOCKNUMBER in  (* this is not the right syntax for this, just an illustration *)
  builtin lt current_time end_of_life
=>

Somewhat related to issue #384.

[anton-trunov]

Turns out this was a documentation issue because Scilla already has support for this, see Zilliqa/scilla-docs#119.

It can be done like so

scilla_version 0

library Constraint

contract Mortal (end_of_life : BNum)
with
  builtin blt _creation_block end_of_life
=>

(* the rest of the contract *)