* src/parse_simulation.c: Fix bug in parse_state_free.
Free new_root when simulate_reduction generates zero states.
* src/parse-simulation.c, src/parse-simulation.h
(parse_state_list, parse_state_list_append): New.
* src/parse-simulation.c, src/parse-simulation.h,
* src/counterexample.c: Replace all uses of lists of parse states and
appends to parse_state_lists with the new API.
* src/derivation.h, src/derivation.c: Make derivation struct opaque.
Add derivation_list type for clarity.
(derivation_list_new): New.
(derivation_list_append): New.
(derivation_list_prepend): New.
(derivation_new_leaf): New constructor for derivations with no
children.
* src/counterexample.c, src/parse-simulation.c,
* src/parse-simulation.h: Replace uses of gl_list_t containing
derivations with derivation_list and its API.
Replace calls of dervation_new using null children with
derivation_new_leaf.
* src/parse-simulation.c: replace ps_chunk and its API with typed
versions si_chunk and deriv_chunk.
* src/parse-simlation.h, src/parse-simulation.c: Remove
parse_state_retain_deriv in favor of derivation reference counting.
* src/counterexample.c: Remove search_state_retain_deriv.
