Implement device-side event delivery.

Finalizes #184 and includes fix for #285. This PR constitutes a brutal rebase of the `devel/async-integration` branch onto current master: git patch, baby.

Implements asynchronous integration of cells within an `fvm_multicell` instance together with the code to implement efficient device-side event delivery.

Summarized patch notes from the development branch:

* Add four new views in mechanism state:
   * `vec_ci`: cv index to cell index (w.r.t. one lowered multicell instance) map.
   * `vec_t`: current or integration starting time, indexed by cell.
   * `vec_t_to`: integration stopping time, indexed by cell.
   * `vec_dt`: pre-computed delta between `vec_t` and `vec_t_to`, to address performance regression on multicore backend.
* Extend notion of indexed view in `modcc` to encompass `CellIndexedVariable` variables, which have a per-cell value.
* Add/change 'built-in' variables `t`, `t_to`, and `dt` for NMODL.
* Additional utility functions:
   * `util...