Properly implement handler::update_host

[fknorr]

The function was not callable so far, it simply referenced an unimplemented function on accessor of the same name.

Calling buffer_state::write_back requires the state to be available inside accessor, which is why I refactored buffer to internally expose its state rather than pointers and ranges individually. This in turn needs type erasure on Allocator, which I quickly added as well.