lisp: split memory into conses and vectors

During garbage collection, we'll iterate through these two vectors.
Instead of freeing the memory, we can swap cells in the vector and
decrement its size.

The idea is we can attach an allocator to the system where we reuse
memory instead of just allocating everytime.

include/alisp/lisp.h

@@ -26,7 +26,8 @@ typedef struct
 /// System context
 typedef struct
 {
-  vec_t memory;
+  vec_t conses;
+  vec_t vectors;
   sym_table_t symtable;
 } sys_t;