Add arena buckets to ArenaPool

ArenaPool previously maintained up to 512 16KB buckets. The 16KB retention is
small for things like XHR and scripts, but increasing it to something more
reasonably, like 128KB, would use up to 8x more memory.

This commit adds 4 buckets: 1KB, 4KB, 16KB and 128KB. Callers can request a
tiny, small, medium or large bucket. We end up using less memory peak memory
and less allocations.

Furthermore, callers can request a specific size. This is particularly useful
for WebSocket or Blob where the size could vary greatly (so we'd likely default
to a large bucket), but that could needlessly use up a large arena.

The bucket sizes were derived from analyzing allocations. A significant number
of allocations were very small. Things like ScheduleCallback and
FinalizerCallback are always less than 1K and can be generated in the thousands.
The 16KB retention was wasteful in these cases...better to have a large number
of 1K pools, so that we can have a handful of very large buffers.

src/App.zig

@@ -69,7 +69,7 @@ pub fn init(allocator: Allocator, config: *const Config) !*App {
     app.telemetry = try Telemetry.init(app, config.mode);
     errdefer app.telemetry.deinit(allocator);
 
-    app.arena_pool = ArenaPool.init(allocator, 512, 1024 * 16);
+    app.arena_pool = ArenaPool.init(allocator, .{});
     errdefer app.arena_pool.deinit();
 
     return app;