resumable should have SizeUnknown() so collect and comprehensions work

[ggggggggg]

See docs for iteratorsize. Consider the following, which prints "second worked" for me.

using ResumableFunctions

@resumable function fibonnaci(n::Int) :: Int
  a = 0
  b = 1
  for i in 1:n-1
    @yield a
    a, b = b, a+b
  end
  a
end

a = fibonnaci(10)

c=try 
  b = fibonnaci(10)
  c=collect(b)
  println("first worked")
  c
catch
  Base.iteratorsize(::Type{typeof(a)}) = Base.SizeUnknown()
  b = fibonnaci(10)
  c=collect(b)
  println("second worked")
  c
end   

@show c

SizeUnknown() would be a strict improvement, but it seems like it would be possible to define length and size correctly for some iterators, such as the fibonnaci example, but it would require extra syntax.

[ggggggggg]

using ResumableFunctions

@resumable function fibonnaci(n::Int) :: Int
  a = 0
  b = 1
  for i in 1:n-1
    @yield a
    a, b = b, a+b
  end
  a
end
Base.iteratorsize(::Type{T}) where T<:ResumableFunctions.FiniteStateMachineIterator = Base.SizeUnknown()
a = fibonnaci(10)
collect(a)

is a better way to do this.