`consume` never returns if task function doesn't contain `produce`

[yebai]

The following code would halt

using Libtask

function f_ct()
   t = 0;
   while t<3
     println(t)
     t = 1 + t
   end
   return t
end

t = CTask(f_ct)

julia> consume(t) # Never returns. 
0
1
2

This is likely due to the consumer task never gets waken up if the producer task exit before any produce statement. Maybe a better behaviour for this kind of tasks is to wake up the producer task with the producer task's return value?

[devmotion]

As a comparison, the following happens with a Channel-based implementation:

The first consumer-producer example in the README could be implemented as

julia> chnl = Channel(; spawn=true) do ch
           t = 0
           while true
               put!(ch, t)
               t += 1
           end
       end;

julia> isempty(chnl)
false

julia> take!(chnl)
0

julia> take!(chnl)
1

The example in this issue gives

julia> chnl = Channel(; spawn=true) do ch
          t = 0
          while t < 3
            println(t)
            t = 1 + t
          end
          return t
       end;

julia> 0
1
2

julia> isempty(chnl)
true

julia> take!(chnl)
ERROR: InvalidStateException("Channel is closed.", :closed)
Stacktrace:
 [1] check_channel_state at ./channels.jl:167 [inlined]
 [2] take_unbuffered(::Channel{Any}) at ./channels.jl:403
 [3] take!(::Channel{Any}) at ./channels.jl:381
 [4] top-level scope at REPL[69]:1

I.e., the function runs once when the channel is constructed and the channel is closed afterwards.

[yebai]

Will be fixed by #93