Error when calling a procedure using a side effect

[PMunch]

This example will create an error on runtime:

import maybe/maybe

var crash = false

proc wontwork(): Maybe[int] =
  if crash:
    nothing[int]()
  else:
    crash = true
    just(4)

maybeCase wontwork():
  just x:
    var y = 3
    echo $(x+y)
  nothing:
    echo "no value"

The reason is simple, maybeCase substitutes all occurences of x with it's first argument. This means that the above get's turned into something like this:

if wontwork().valid:
  var y = 3
  echo $(wontwork().value + y)
else:
  echo "no value"

As we can see wontwork is called twice, the second time producing a different result than the first. What should be happening is something like this:

let x = wontwork()
if x.valid:
  var y = 3
  echo $(x.value + y)
else:
  echo "no value"

[PMunch]

Fixed in #10

[superfunc]

Noted in the PR, will take a look this weekend, thanks!