Added inter-task communication channels

[amitmurthy]

This PR implements only the inter-task (not inter-process) channels part of #12042

The default channel (of size 32) is faster than produce-consume as seen below.

julia> function foo(t::Task)
           while true
               consume(t)
           end
       end;

julia> function foo(n::Int)
           self = current_task()
           @schedule foo(self)
           for i in 1:10^n
               produce(i)
           end
       end;

julia> foo(1);

julia> @time foo(6);
   2.226 seconds      (2005 k allocations: 31484 KB, 0.17% gc time)

julia> function bar(c::Channel)
           while true
               take!(c)
           end
       end;

julia> function bar(n, c)
           @schedule bar(c)
           for i in 1:10^n
               put!(c, i)
           end
       end;

julia> bar(1, Channel())

julia> @time bar(6, Channel())
 104.415 milliseconds (1125 k allocations: 21483 KB, 2.48% gc time)                                                                                                                      

julia> bar(1, Channel(1))

julia> @time bar(6, Channel(1))
   2.287 seconds      (5000 k allocations: 198 MB, 0.74% gc time)                                                                                                                        ```

[amitmurthy]

Any objections to merging this? Both the feature and code is pretty straightforward and doesn't break any existing stuff.

I'll rebase #12042 if/when this gets merged. That will make is easier to review #12042 and we can take a call if we want to go head with the relatively major change of #12042.

[jakebolewski]

I think this should throw an error rather the silently truncate. I feel the user should also be able. To define a Chanel length greater than 32.

I guess you could also throw a nicer error checking if the Chanel size is greater than or equal to 1.

[amitmurthy]

You can define a channel size greater than 32. data is a circular buffer that is managed via szp1, take_pos and put_pos. The initial allocation is 32, It is grown on demand to a maximum size of sz_max. Even a channel of typemax(Int) will only allocate storage for 32 objects initially.

[amitmurthy]

A further optimization that should be done is halving data once the current number of objects is less than 50% of data length

[amitmurthy]

I'll merge this in a day or two if there are no further issues.