[TCLCORE] Coroutines in HEAD

[miguel <ms...@us...>]

I managed to commit an experimental implementation of coroutines in time for 
8.6a2. This provides two new commands
   ::tcl::unsupported::coroutine
   ::tcl::unsupported::yield

A brief description can be found at http://msofer.com:8080/wiki?name=Coroutines, 
otherwise the code in tests/unsupported.test is the only documentation.

Test! Enjoy! Help make it better

Cheers
Miguel