Add a way to extract work result from a thread

[marekjm]

As in the title. Machine should provide a way of extracting return values from finished threads.

If an exception has been thrown but has not been handled in the child thread, extracting return value would rethrow it inside parent thread. To prevent the exception from being rethrown user can send the terminated/1 message to a thread to check if it has been terminated.

[marekjm]

This will require amending thjoin instruction. It should take two operands instead of one:

1. the index of return register (the return value of a thread is return value of a top-most function),
2. index of register containing handle of the thread to join,

Code:

; old syntax
thjoin <thread-handle>

; new syntax
thjoin <return-register> <thread-handle>