By storing it as size_t we explicitly disallow negative line numbers,
and keep 0 as an error/empty/undefined location.
A small problem with this is that MessageContainer.error and friends
still accept negative numbers (often without warning) via size_t
implicit conversion and wrap-around.
While the implementation is a simple alias to
std::vector<Message>::const_iterator, it enablese generic lookup of the
iterator type at compile time, and enables us to effortlessly change the
underlying implementation later.
By supporting operator bool() we get the same existence checking
capabilities as the unique_ptr implementations, without the hurdles of
non-copyability and manual heap allocation.
