Unexpected behavior when using custom finalizers + ARC/ORC + RootObj

[giaco]

I was tracking down a memory issue when I encountered an unexpected the 
behavior of ORC/ARC when using custom finalizers.

I'm using `new` function to assign a finalizer to a type. 
<https://nim-lang.org/docs/system.html#new%2Cref.T%2Cproc%28ref.T%29>

Not sure if this is worth a github issue, please let me know. Here a small test 
to expose the problem.

Compile with --gc:arc
    
    
    const n = 0
    
    when n == 0: # RootObj + forward declaration
      # Custom finalizer not called
      type Foo = ref object of RootObj
      proc delete(self: Foo)
      proc newFoo: Foo = new(result, delete)
      proc delete(self: Foo) = echo("delete Foo")
    
    when n == 1: # RootObj + no forward declaration
      # Working correctly
      type Foo = ref object of RootObj
      proc delete(self: Foo) = echo("delete Foo")
      proc newFoo: Foo = new(result, delete)
    
    when n == 2: # no RootObj + forward declaration
      # Error: internal error: expr: proc not init delete
      type Foo = ref object
      proc delete(self: Foo)
      proc newFoo: Foo = new(result, delete)
      proc delete(self: Foo) = echo("delete Foo")
    
    when n == 3: # no RootObj + no forward declaration
      # Working correctly
      type Foo = ref object
      proc delete(self: Foo) = echo("delete Foo")
      proc newFoo: Foo = new(result, delete)
    
    if isMainModule:
      discard newFoo()
    
    
    Run