Cancellation improvements and some fixes

* bumped version to 3.7.8
* lane:cancel() now accepts a boolean second argument when soft
cancelling (negative timeout) to wake the thread if necessary
* if a blocked linda send() or receive() call is interrupted by a
cancellation request, it returns CANCEL_ERROR so that this case can be
differentiated from a simple timeout
* fixed WIN32 THREAD_CREATE() wrong _beginthreadex() error detection
* fatal WIN32 threading errors retrieve and output the error description
string with FormatMessage()
* fixed missing lanes.set_singlethreaded
* fixed perftest.lua
* added test/cancel.lua