Remove custom-arenas, use ArenaPool instead

This removes the browser-specific arenas (session, transfer, page, call) in
favor of the arena pool.

This is a bit of a win-lose commit. It exists as (the last?) step before I can
really start working on frames. Frames will require their own "page" and "call"
arenas, so there isn't just 1 per browser now, but rather N, where N is the
number of frames + 1 page. This change was already done for Contexts when
ExecutionWorld was removed, and the idea is the same: making these units more
self contained so to support cases where we break out of the "1" model we
currently have (1 browser, 1 session, 1 page, 1 context, ...).

But it's a bit of a step backwards because the ArenaPool is dumb and just resets
everything to a single hard-coded (for now) value: 16KB. But in my mind, an
arena that's used for 1 thing (e.g. the page or call arenas) is more likely to
be well-sized for that specific role in the future, even on a different
page/navigate.

I think ultimately, we'll move to an ArenaPool that has different levels, e.g.
acquire() and acquireLarge() which can reset to different sizes, so that a page
arena can use acquireLarge() and retain a larger amount of memory between use.

src/ArenaPool.zig

@@ -85,3 +85,8 @@ pub fn release(self: *ArenaPool, allocator: Allocator) void {
     self.free_list_len = free_list_len + 1;
     self.free_list = entry;
 }
+
+pub fn reset(_: *const ArenaPool, allocator: Allocator, retain: usize) void {
+    const arena: *std.heap.ArenaAllocator = @ptrCast(@alignCast(allocator.ptr));
+    _ = arena.reset(.{ .retain_with_limit = retain });
+}