Add a future data type for concurrent and parallel programming

library/library.m:
library/thread.future.m:
library/thread.m:
    Add new future standard library module.

NEWS:
    Announce the new addition.

library/thread.semaphore.m:
    Add an impure interface to thread.semaphore.m.  Semaphores are used to
    implement our other concurrency primitives and an impure interface can
    often be useful to implement things such as futures, which don't require
    IO state threading.  The impure interface predicate names are prefixed
    with "impure_".

library/thread.semaphore.m:
NEWS:
    Deprecate the impure init/1 function.

library/thread.mvar.m:
    Conform to changes in semaphore.m.

benchmarks/progs/mandelbrot/mandelbrot.m:
    Add future example to mandelbrot benchmark.
Add an impure_init/0 function to thread.mvar.m

This change renames the init/0 function in thread.mvar.m to impure_init/0;
deprecating the old function.  This makss this module more consistent with
thread.semaphore.m.

Remove the "mvar." prefix where it is unnecessary.

Add %----% separators between some definitions in the implementation
section.

library/thread.mvar.m:
    As above.