Safe std.parallelism

[dsimcha]

As I've mentioned before, my std.parallelism module (formerly parallelfuture;
http://cis.jhu.edu/~dsimcha/d/phobos/std_parallelism.html,
http://dsource.org/projects/scrapple/browser/trunk/parallelFuture/std_parallelism.d)
is currently in review for inclusion in Phobos.

Originally I thought that, for a pedal-to-metal parallelism library, any safe
construct would be too limited to be useful to anyone.  Based on some
discussions on this NG and the Phobos mailing list I'm starting to change my
mind.  I think std.parallelism should still allow, and even be primarily
geared towards pedal-to-metal parallelism, but I think map(), reduce() and
Task could be made safe under some limited but useful circumstances.

I'd like to bounce a design question off the community.  Given the current
design of std.parallelism, I see three ways of adding the safe functionality.

1.  Create safeTask(), safeMap(), safeReduce() that only accept safe
arguments.  This is ugly but explicit.  Suggestions for better names would be
appreciated.

2.  Just overload task(), map() and reduce() so that the overload marked
@trusted can be used in SafeD and is as safe as std.concurrency, and the
unsafe version can be called from @system code.

3.  Name the safe versions task(), map() and reduce() and rename the unsafe
versions something else.  (I kind of don't like this because std.parallelism,
by its nature, is mostly geared towards unsafe, pedal-to-metal parallelism.)