refactor(arena): use a shared block freelist

This is both simpler in client code and more effective (always reuse
block hottest in cache)

src/nvim/msgpack_rpc/channel.c

@@ -158,7 +158,7 @@ Object rpc_send_call(uint64_t id, const char *method_name, Array args, ArenaMem
     }
 
     // frame.result was allocated in an arena
-    arena_mem_free(frame.result_mem, &rpc->unpacker->reuse_blk);
+    arena_mem_free(frame.result_mem);
     frame.result_mem = NULL;
   }
 
@@ -244,7 +244,7 @@ static void parse_msgpack(Channel *channel)
         ui_client_event_raw_line(p->grid_line_event);
       } else if (p->ui_handler.fn != NULL && p->result.type == kObjectTypeArray) {
         p->ui_handler.fn(p->result.data.array);
-        arena_mem_free(arena_finish(&p->arena), &p->reuse_blk);
+        arena_mem_free(arena_finish(&p->arena));
       }
     } else if (p->type == kMessageTypeResponse) {
       ChannelCallFrame *frame = kv_last(channel->rpc.call_stack);
@@ -295,7 +295,7 @@ static void handle_request(Channel *channel, Unpacker *p, Array args)
   if (!p->handler.fn) {
     send_error(channel, p->type, p->request_id, p->unpack_error.msg);
     api_clear_error(&p->unpack_error);
-    arena_mem_free(arena_finish(&p->arena), &p->reuse_blk);
+    arena_mem_free(arena_finish(&p->arena));
     return;
   }
 
@@ -364,7 +364,7 @@ static void request_event(void **argv)
 
 free_ret:
   // e->args (and possibly result) are allocated in an arena
-  arena_mem_free(arena_finish(&e->used_mem), &channel->rpc.unpacker->reuse_blk);
+  arena_mem_free(arena_finish(&e->used_mem));
   channel_decref(channel);
   xfree(e);
   api_clear_error(&error);

src/nvim/msgpack_rpc/unpacker.c

@@ -181,13 +181,11 @@ void unpacker_init(Unpacker *p)
   p->unpack_error = (Error)ERROR_INIT;
 
   p->arena = (Arena)ARENA_EMPTY;
-  p->reuse_blk = NULL;
 }
 
 void unpacker_teardown(Unpacker *p)
 {
-  arena_mem_free(p->reuse_blk, NULL);
-  arena_mem_free(arena_finish(&p->arena), NULL);
+  arena_mem_free(arena_finish(&p->arena));
 }
 
 bool unpacker_parse_header(Unpacker *p)
@@ -308,7 +306,7 @@ bool unpacker_advance(Unpacker *p)
       p->state = 10;
     } else {
       p->state = p->type == kMessageTypeResponse ? 1 : 2;
-      arena_start(&p->arena, &p->reuse_blk);
+      p->arena = (Arena)ARENA_EMPTY;
     }
   }
 
@@ -322,7 +320,7 @@ bool unpacker_advance(Unpacker *p)
       goto done;
     } else {
       // unpack other ui events using mpack_parse()
-      arena_start(&p->arena, &p->reuse_blk);
+      p->arena = (Arena)ARENA_EMPTY;
     }
   }
 
@@ -416,13 +414,13 @@ redo:
     if (p->ui_handler.fn != ui_client_event_grid_line) {
       p->state = 12;
       if (p->grid_line_event) {
-        arena_mem_free(arena_finish(&p->arena), &p->reuse_blk);
+        arena_mem_free(arena_finish(&p->arena));
         p->grid_line_event = NULL;
       }
       return true;
     } else {
       p->state = 13;
-      arena_start(&p->arena, &p->reuse_blk);
+      p->arena = (Arena)ARENA_EMPTY;
       p->grid_line_event = arena_alloc(&p->arena, sizeof *p->grid_line_event, true);
       g = p->grid_line_event;
     }

src/nvim/msgpack_rpc/unpacker.h

@@ -32,8 +32,6 @@ struct Unpacker {
   Error unpack_error;
 
   Arena arena;
-  // one length free-list of reusable blocks
-  ArenaMem reuse_blk;
 
   int nevents;
   int ncalls;