Closes #1488
This PR implements support for binary operations in constants.

A few notes:

• It's still missing tests. This probably needs a lot of tests.

• This is implemented differently on Erlang and Javascript, due to the different languages being somewhat different.
  For example, this code:

  const minutes_per_hour = 60
  const hours_per_day = 24
  const minutes_per_day = minutes_per_hour * hours_per_day
  
  pub fn main() {
    minutes_per_day
  }

  Compiles to this Javascript:

  const minutes_per_hour = 60
  const hours_per_day = 24
  const minutes_per_day = minutes_per_hour * hours_per_day
  
  export function main() {
    return minutes_per_day;
  }

  And this erlang:

  main() ->
      1440.

  Although this constant folding process could be done in the analysis stage instead and be applied to javascript as well.

• I've been having some trouble trying to get the formatter to play nicely with this; I still don't really understand how it works. If someone who is more familiar with it could point me in the right direction, that would be amazing.

• I'm not super happy with the parser code for constant binary operations: I had to copy a lot of the expression-parsing code. It might be good to merge this code somehow.