Replace `util::either` with `util::expected`. (#1142)

* Implement a workalike for the proposed `std::expected` class: see http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p0323r9.html .
* Replace use of `either` with `expected` in `mprovider`, `mechanism_catalogue`, `util::partition_range`, and `pyarb::hopefully`.
* Replace use of `either` with `variant` in `util::sentinel_iterator`.
* Add `in_place_t` constructor for `util::optional`.
* Fix move assignment bug in `util::variant`.
* Remove `util/either.hpp` and associated tests.

Fixes #1135.