Introduces a ArenaAllocator wrappre (lp.Arena). This is groundwork for better
memory tracking and reporting memory usage to v8. This is almost purely a
mechanical change to lay a foundation for a follow up PR that will address
https://github.com/lightpanda-io/browser/issues/3027
Some code became a bit leaner: a pooled arena can release itself (it has a
reference to the ArenaPool).
Some code became uglier: The Frame has a `_local_arena: *lp.Arena` and a
`local_arena: Allocator` (same with call_arena, and same with a few other types)
so that consumers aren't impacted (they continue to use `frame.local_arena`).
Halil Durak