aklisp.h: Add feature handling

    o Rename li_elem_count field to li_count
    o Add comments on other fields
lib.c: Add new functions (akl, length, ...)
modules/hello/hello.c: Use the reworked module format
src/main.c: Use the new features to show debug informations
list.c: Add the new list iterators
examples/*.c: Add C examples (for library usage of AkLisp)
parser.c: Add akl_str_to_list() function
tests/list.c: Use the new akl_list_count() function
tests/vector.c: Test vector's vector_is_empty() function
*Makefile*: Work started for a simple build environment
modules/hello/hello.c: Use the new module definition API
aklisp.h: Reformat declarations

    * Create struct akl_mem_callbacks from struct akl_state's
      allocation function pointers
    * Rewrite akl_new_*(), akl_init_state(), etc. functions to
      use the new memory callbacks
    * Add AKL_ASSERT() for debug and run-time assertation
    * Add some new list iterators
    * And some more...
examples/love_ny.lsp: Remove
aklisp.c: Use the new APIs

Work in progress on the stack handling
compile.c: Add IR builder for get, set instructions

    * Use AKL_ASSERT()
gc.c: Define akl_mem_std_callbacks

    * Use the s->ai_mem_fn pointers
    * Add some AKL_ASSERT()
    * Define akl_set_mem_callbacks()
    * Use the new AKL_*_FEATURE*() APIs for configuration
gc.c: No need for integer overflow fix

The passed calloc(), must have to deal with it
lexer.c: Fix comment typo
lib.c: Add the about library function

    * Rename akl-set! to akl-cfg! improve show_features() to use
     the new name (fix it's location too in the comment)
lib_spec.c: Add set, when special library functions

   * Rename if, while to sif, swhile in the C code to avoid
     keyword conflict
   * Define the new list iterators