Modified Prisoner's Dilemma lab.

Sparked by
http://lesswrong.com/lw/7f2/prisoners_dilemma_tournament_results/4ru9

A match brings together two programs written in a custom dialect of
Scheme. Each is called with two inputs: the source code of itself and
the other. (Giving it its own source code is only a convenience, of
course.) It's allowed to run for some standard number of steps
('fuel'). The output should be 'C or 'D within the allotted time. If
not, both programs earn 0. Otherwise, standard prisoner's-dilemma
payoffs.

In a round-robin tournament, agents get ranked by total score over all
matches.

Implementation in Gambit Scheme using its WITH-EXCEPTION-CATCHER 
primitive; I believe it's portable Scheme otherwise.

(RUN fuel expression environment) is like the standard EVAL but with
limited fuel, and returning a list like (unconsumed-fuel
result-value). In the scope of this run, (GLOBAL-ENVIRONMENT) returns
the environment passed in. The result in the case of an error or fuel
exhaustion is defined in terp.scm -- I may want to change all these
particulars.

To implement Eliezer's 

> This enables you to say, "Simulate my opponent, and if it tries to
> simulate me, see what it will do if it simulates me outputting
> Cooperate."

rebind RUN to your preferred function in the environment passed to RUN.
(For convenience a REBIND function is supplied.)

;; Example agents

;; Trivial cooperator and defector
(lambda (me them) 'C)
(lambda (me them) 'D)

;; Cooperate with shallow, cooperative agents (first cut)
(lambda (me them)
  (let ((result (run 1000
                     (list them them me)
                     (rebind 'run run  ;TODO: interpose a new RUN
                             (global-environment)))))
    (let ((remaining (car result))
          (value (cadr result)))
      (if (< 500 remaining)
          (if (equal? value 'C)
              'C
              'D)
          'D))))