Removes a boost header from the public includes in favor of using the
standard-provided std::variant.
The use of boost in public interfaces is often a dealbreaker for some
people. Given we use std::optional in the header already, we can
transition over to std::variant from boost::variant.
With this removal, this makes all of our dependencies internal to the
library itself.
Provides a more "correct" move constructor/assignment operator, since
these relevant functions shouldn't throw exceptions.
Has the benefit of playing nicely with std::move_if_noexcept and other
noexcept library facilities.
