Add LISP interpreter

tool/plinko/README.txt

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+DESCRIPTION
+
+  plinko is a simple lisp interpreter that takes advantage of advanced
+  operating system features irrespective of their practicality such as
+  using the nsa instruction popcount for mark sweep garbage collection
+  overcommit memory, segment registers, and other dirty hacks that the
+  popular interpreters cannot do; this lets plinko gain a considerable
+  performance edge while retaining an event greater edge in simplicity
+
+  We hope you find these sources informative, eductional, and possibly
+  useful too. Lisp source code, written in its dialect is included too
+  under //tool/plinko/lib and unit tests which clarify their usage can
+  be found in //test/tool/plinko.
+
+BENCHMARK
+
+  binary trees (n=21)
+
+    - sbcl:     200 ms (native jit;  simulated arithmetic)
+    - plinko:   400 ms (interpreted; simulated arithmetic)
+    - python3:  800 ms (interpreted;    native arithmetic)
+    - racket:  1200 ms (interpreted; simulated arithmetic)
+
+AUTHOR
+
+  Justine Alexandra Roberts Tunney <jtunney@gmail.com>
+
+LICENSE
+
+  ISC
+
+SEE ALSO
+
+  SectorLISP
+  SectorLambda