Pool allocator - #4

Some changes were merged into this 'garbage-collection' branch in #4.

Major changes:

* Move memory-related functions to 'memory.c' (this should have been done in
  main).
* Add pool allocator for expressions ('Expr' structures).
* Add garbage collector that marks expressions from the pool, and frees the
  non-marked ones.
* Replace 'expr_free' with 'pool_free', remove 'expr_list_free'.

Minor changes:

* Remove most calls to 'expr_free', only call manually from 'parse' (other calls
  are currently from the garbage collector or the pool itself).
* Don't use stack variables from 'env_init_defaults' (and therefore from the
  'BIND_PRIM_FLAGS' macro).
* Rename 'tru' and 'nil' C globals to 'g_tru' and 'g_nil' (this should have been
  done in 'main', after merging).
* Add 'g_debug_trace_list' global variable to 'env.c'.
* Add 'CPPFLAGS' to Makefile (this should have been done in 'main', after
  merging).
* Improve documentation of 'EExprType' enum and 'Expr' structure in comments.
* Add many TODO comments.

Changes not included in this PR (currently in a 'copy-references' branch):

* Use references instead of clones in most places.

Work with references, instead of always returning copies. This change was
possible thanks to #5 (which needed #4), #6 and #7. Some changes were merged
into this 'copy-references' branch in #7.

Major changes:

* Remove most calls to 'expr_clone', for working with references instead of
  copies.
* Return or store references in 'car', 'cdr', 'cons', 'nth', 'quote' and
  'backquote' primitives.
* Add proper environment closures, setting the parent environments whenever a
  lambda is created, not when it's called. See #7.
* Change how GC handles lambdas, add 'is_used' member to 'Env'. Mostly included
  in #7, but not completely (see 5758c96).
* Don't mark globals ('g_nil', 'g_tru' and 'g_debug_trace_list') for garbage
  collection, since their references are being stored in the environment.

Minor changes:

* Add 'gc_mark_env' and 'gc_mark_env_and_parents' static functions to
  'garbage_collector.c'.
* Add an 'Env*' parameter to 'lambdactx_init', remove it from
  'lambdactx_eval_body'.
* Don't make the 'Expr*' parameter of 'env_bind' and 'env_bind_global' constant.
* Add closure test to 'test/lambdas.lisp'. See #7.