add our own malloc implementation (just a mock for now)

include/picrin.h

@@ -6,6 +6,10 @@
 typedef struct {
 } pic_state;
 
+void *pic_alloc(pic_state *, size_t);
+struct pic_object *pic_gc_alloc(pic_state *, size_t, enum pic_tt);
+void pic_free(pic_state *, void *);
+
 pic_state *pic_open();
 void pic_close(pic_state *);