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.

test/test_lisp_api.c

@@ -151,44 +151,42 @@ void sys_test(void)
   TEST_START();
   sys_t sys = {0};
   sys_init(&sys);
-  u64 old_memory_size = sys.memory.size;
+  u64 old_memory_size = sys_cost(&sys);
 
   // Creating integers doesn't affect memory size
   (void)make_int(2000);
-  TEST(sys.memory.size == old_memory_size,
+  TEST(sys_cost(&sys) == old_memory_size,
        "Making integers doesn't affect system memory size");
 
-  // Creating symbols won't affect memory size, but does affect the symbol table
+  // Creating symbols does affect memory size and memory table
   (void)intern(&sys, SV_AUTO("hello world!"));
-  TEST(sys.memory.size == old_memory_size,
+  TEST(sys_cost(&sys) > old_memory_size,
        "Interning doesn't affect system memory size");
   TEST(sys.symtable.count > 0, "Interning affects symbol table");
+  old_memory_size = sys_cost(&sys);
 
   // Creating conses do affect memory size
   (void)cons(&sys, make_int(1), make_int(2));
-  TEST(sys.memory.size > 0, "Creating conses affects memory size");
-  old_memory_size = sys.memory.size;
+  TEST(sys_cost(&sys) > 0, "Creating conses affects memory size");
+  old_memory_size = sys_cost(&sys);
 
   (void)cons(&sys, intern(&sys, SV_AUTO("test")), NIL);
-  TEST(sys.memory.size > old_memory_size,
+  TEST(sys_cost(&sys) > old_memory_size,
        "Creating conses back to back affects memory size");
-  old_memory_size = sys.memory.size;
+  old_memory_size = sys_cost(&sys);
 
   // Creating vectors does affect memory size
   (void)make_vec(&sys, 8);
-  TEST(sys.memory.size > old_memory_size,
+  TEST(sys_cost(&sys) > old_memory_size,
        "Creating vectors (size 8) affects memory size");
-  old_memory_size = sys.memory.size;
+  old_memory_size = sys_cost(&sys);
 
   (void)make_vec(&sys, 1000);
-  TEST(sys.memory.size > old_memory_size,
+  TEST(sys_cost(&sys) > old_memory_size,
        "Creating vectors (size 1000) affects memory size");
-  old_memory_size = sys.memory.size;
+  old_memory_size = sys_cost(&sys);
 
   sys_free(&sys);
-  TEST(sys.memory.size == 0, "sys_free cleans up memory (shallow check)");
-  TEST(sys.symtable.count == 0, "sys_free cleans up symtable (shallow check)");
-
   TEST_END();
 }