Feature/lib install target part 3 (#518)

This time we're moving `recipe.hpp` and `simulation.hpp`, plus the requirements they bring.

Code changes:
* Pimplize `simulation`.
* Consolidate arbor exceptions: all non-cell kind specific exceptions that might be expected to reach user code now have consistent messages and fit in an exception hierarchy based at `arb::arbor_exception`. Internal errors throw an `arb::arbor_internal_error` exception.
* Renamed `postsynaptic_spike_event` to `spike_event`. (Note: `pse_vector` name is unchanged.)
* Repurposed `pprintf` and moved it into `strprintf.h` — further consolidation is a TODO.
* Made a generic `util::to_string` to avoid redundancy of `operator<<` overloads and other `to_string` definitions. Defaults to ADL `to_string`, `std::to_string`, and finally tries using `operator<<`.