Advanced threads and actors

Turn threads into possible expressions, an extremely handy feature. Possibly introduce actors from actress for communication with other threads during their execution:

// thread syntax TBD
let mynum = spawn || {
    stuff1();
    stuff2();
    etc();
    4
}

// Do other stuff
let x = 4 + mynum; // mynum is joined now and it's return value is taken
// after this point mynum is a regular integer variable
// mynum.join() for manual joining, noop if already joined

Start with the proposed thread syntax in the issue body, including spawn, implicit joining through mynum, and mynum.join(), then investigate how actor communication via actress would fit. Before implementation, clarify the syntax, execution semantics, and scope of actor support; done would require an agreed design rather than the current tentative proposal.