A CESK machine-style evaluator for LambdaS5

Added:
  src/ljs/ljs_cesk.ml
  - main CESK evaluate function and entry point from s5.ml
  src/ljs/ljs_clos.ml
  - machine-specific closures and functions on closures
  src/ljs/ljs_kont.ml
  - machine-specific continuations and functions on continuations
  tests/single_test.sh
  - easily run single tests for with CESK or S5 eval

Modified:
  src/s5.ml
  - added -eval-cesk and -continue-cesk-eval evaluation flags

Currently passing 208 tests while failing 4--parity with ljs_eval.

Future work:
- improve our naive garbage collection strategy
- develop this evaluator into an analyzer using the "abstracting
  abstract machines" approach (Van Horn and Might, CACM'11)