Order synapse compartment indices.

Present an ordered sequence of compartment indices
to point processes (synapses) on creation in fvm_multicell.
Required for GPU implementations.

Also:
* Change target handle for fvm_multicell to refer to actual
  (rather than offset) mechanism index.
* Split out the range/sequence utilities that were accumulating
  in `util/range.hpp` into their own header `util/rangeutil.hpp`.
  Possibly `sequtil.hpp` is a better name…
* Add range/sequence-friendly `util::sort` and `util::sort_by`.
  The latter sorts based on comparing a derived key.
* Add `util::assign_by`. Assigns after applying a transformation.
* Fix `canonical_view` for non-const ranges/sequences.
* Unit tests for `assign`, `assign_by`, `sort`, `sort_by`.