Fix Context's call_arena

The Context's call_arena should be based on the source, e.g. the IsolateWorld
or the Page, not always the page. There's no rule that says all Contexts have
to be a subset of the Page, and thus some might live longer and by doing so
outlive the page_arena.

Also, on context cleanup, isolate worlds now cleanup their identity.

src/ArenaPool.zig

@@ -112,7 +112,6 @@ pub fn acquire(self: *ArenaPool, dbg: DebugInfo) !Allocator {
         }
         gop.value_ptr.* += 1;
     }
-
     return entry.arena.allocator();
 }