Deinit and arena allocator

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You can easily make free a no-op:

fn my_free (_: *anyopaque, _: []u8, alignment: Alignment, _: usize) void {}
const my_vtable = blk:{
   // If we could just grab the functions
   // inside the Arena implementation, 
   // we could easily create a new vtable 
   // with free replaced. But these functions
   // are private, so we do a little dance to
   // grab them.
   var arena: std.heap.ArenaAllocator = undefined;
   const allocator = arena.allocator();
   var vtable = allocator.vtable;
   vtable.free = my_free;
   break :blk vtable;
};

fn myArenaAllocator(arena: *std.heap.ArenaAllocator) Allocator{
  var allocator = arena.allocator();
  allocator.vtable = &my_vtable;
  return allocator;
}
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