Reduce differences between `util::optional` and `std::optional`. (#420)

Reduce differences between `util::optional` and `std::optional`.

* Rename `util::nothing` to `util::nullopt`.
* Replace `util::get()` by`util::optional::value()`.
* Add correct move semantics to `util::optional::value()`.
* Add `util::optional::value_or()` method.
* Remove unused monadic functionality from `util::optional` and `util::uninitialized`.
* Update code formatting to closer adhere to coding guidelines.
* Add convenience ""_s string constructor for unit tests.
* Update unit tests accordingly.

Fixes #419.