Prepare some code for the new memory manager

This commit will introduce the next GC improvements, while fixes some
older typos in the codebase.

The new GC will be commited later on...
src/gc.c: Garbage collector getting more dynamic

GC now supports user-defined types. These types implemented with the
akl_gc_type structure, which is live in the ai_gc_types member in the
akl_state.

Base types automatically added in akl_gc_init().