Pre and post for variable modification event should only be created when used

[shaunazzopardi]

Currently functions to access the previous value and current value of a variable are being created for each variable event, e.g. an event counter@() will result in the following functions being added to the smart contract:

  function LARVA_set_counter_pre (uint counter) ... {
     .....
  }
  function LARVA_set_counter_post (uint counter) ... {
     .....
  }

However this is wasteful. The code in the event condition should be analysed to only create the functions required.