Preliminary work towards the mechanism ABI.

Unification of the `public` and `protected` interfaces of the mechanism classes.
These interfaces will be merged into the public API ('public' as in: exposed via `arbor/include/arbor`). 

Interface methods `nrn_` have been renamed
- `nrn_init`: `init` (would have preferred initialize, but that is taken)
- `deliver_events(event_stream)`: `apply_events` (to avoid a name clash)
- `nrn_state`: `advance_state`
- `nrn_current`: `compute_currents`

Additional changes:
  - Remove typedefs for index_type etc
  - Shuffle `ion_state_view` to public interface
  - Migrate `copy_extend` to a proper place out of `multicore/mechanism.hpp`. Maybe `memory` or `range_util`.
  - Remove `vec_t_to_`