Ranges and views implementations.

* Adds non-owning range class describing a half-open interval
  by a pair of iterators, or by a start iterator and sentinel.
  (Assumption is that these are bounded ranges; unbounded
  ranges are on the todo list.)
* Adds spans and counting iterators, for iterating across
  intervals of integral types with a range-like interface.
* Partition in-place creation functions and partition views
  over monotonically increasing sequences.
* Two-field variant type implementation `util::either<A, B>`.
  (Used to provide an STL- and range for-loop accessible
  façade to sentinel-terminated ranges.)
* Some simplifications of utitliy classes and tidying up
  of formatting.