Lightweight Ruby MRI promise extension
  (c) 2010 Lourens Naudé (methodmissing), James Tucker (raggi), tmm1 && ice799 for pointers (pun intended)
  https://github.com/methodmissing/promise
  ** WORK IN PROGRESS **
This library works with Ruby 1.8 (1.9 pending) and is a more efficient
implementation of the following Ruby code :
  class Promise
    def initialize(&blk)
      @thread = Thread.new(&blk)
      @result = nil
    end
  
    def method_missing(meth, *args)
      @result ||= @thread.value
    end
  end
Examples :
  promise = Promise.new{ 1000.times{ IO.read(__FILE__) } }
  assert_equal 1000, promise.id # blocks only if we don't have a result from the computation thread yet
Challenges :
  * Avoiding method_missing overhead, which bogs down even 1.9's BasicObject for this use case
  * rb_define_method(rb_cPromise, "__send__", rb_promise_result_argc_any, -1);
  * We attempt to negate some of that by overloading common Object members at compile time
  * Ideally one should overload rb_call (block the current thread) and thus a Promise
    implementation is best served as a MRI patch.
Todo :
  * ruby 1.9 support
  * Fiber support
  * reduce further method call overhead
  * look into a symbol table for caching
Performance :
  methodmissing:promise lourens$ rake bench
  (in /Users/lourens/projects/promise)
  /Users/lourens/.rvm/rubies/ruby-1.8.7-p249/bin/ruby bench/promise.rb
  Rehearsal --------------------------------------------------------------------
  Promise                            0.010000   0.000000   0.010000 (  0.018255)
  RubyPromise                        0.030000   0.000000   0.030000 (  0.024190)
  Promise#==                         0.010000   0.000000   0.010000 (  0.017622)
  RubyPromise#==                     0.020000   0.010000   0.030000 (  0.020867)
  Promise#object_id                  0.010000   0.000000   0.010000 (  0.016699)
  RubyPromise#object_id              0.020000   0.000000   0.020000 (  0.021970)
  Promise#__send__                   0.020000   0.000000   0.020000 (  0.017142)
  RubyPromise#__send__               0.020000   0.000000   0.020000 (  0.021984)
  Promise (blocking)                 0.040000   0.030000   0.070000 (  0.072591)
  RubyPromise (blocking)             0.050000   0.030000   0.080000 (  0.077348)
  Promise#== (blocking)              0.050000   0.030000   0.080000 (  0.068011)
  RubyPromise#== (blocking)          0.050000   0.020000   0.070000 (  0.084727)
  Promise#object_id (blocking)       0.050000   0.030000   0.080000 (  0.069918)
  RubyPromise#object_id (blocking)   0.050000   0.030000   0.080000 (  0.082319)
  Promise#__send__ (blocking)        0.040000   0.020000   0.060000 (  0.071780)
  RubyPromise#__send__ (blocking)    0.060000   0.030000   0.090000 (  0.088574)
  ----------------------------------------------------------- total: 0.760000sec
                                         user     system      total        real
  Promise                            0.010000   0.000000   0.010000 (  0.014602)
  RubyPromise                        0.020000   0.010000   0.030000 (  0.022575)
  Promise#==                         0.010000   0.000000   0.010000 (  0.015162)
  RubyPromise#==                     0.020000   0.000000   0.020000 (  0.019323)
  Promise#object_id                  0.010000   0.000000   0.010000 (  0.015996)
  RubyPromise#object_id              0.010000   0.000000   0.010000 (  0.019146)
  Promise#__send__                   0.010000   0.010000   0.020000 (  0.015637)
  RubyPromise#__send__               0.020000   0.000000   0.020000 (  0.019562)
  Promise (blocking)                 0.040000   0.020000   0.060000 (  0.069022)
  RubyPromise (blocking)             0.050000   0.030000   0.080000 (  0.080459)
  Promise#== (blocking)              0.050000   0.030000   0.080000 (  0.068640)
  RubyPromise#== (blocking)          0.050000   0.020000   0.070000 (  0.084409)
  Promise#object_id (blocking)       0.050000   0.030000   0.080000 (  0.067952)
  RubyPromise#object_id (blocking)   0.060000   0.030000   0.090000 (  0.085177)
  Promise#__send__ (blocking)        0.040000   0.020000   0.060000 (  0.067793)
  RubyPromise#__send__ (blocking)    0.060000   0.030000   0.090000 (  0.086539)
To run the test suite:
  rake
To run the benchmarks:
  rake bench