alloc: arena_t -> alloc_t

arena_t doesn't really make sense given we also have a free list.
Better to name it generic.

include/alisp/allocator.h

@@ -34,12 +34,12 @@ typedef struct
 {
   vec_t pages;
   vec_t free_list;
-} arena_t;
+} alloc_t;
 
-lisp_t *arena_make(arena_t *, tag_t type);
-void arena_delete(arena_t *, lisp_t *);
-u64 arena_cost(arena_t *);
-void arena_free(arena_t *);
+lisp_t *alloc_make(alloc_t *, tag_t type);
+void alloc_delete(alloc_t *, lisp_t *);
+u64 alloc_cost(alloc_t *);
+void alloc_free(alloc_t *);
 
 #endif
 

include/alisp/sys.h

@@ -14,7 +14,7 @@
 /// System context
 typedef struct
 {
-  arena_t memory;
+  alloc_t memory;
   sym_table_t symtable;
 } sys_t;