[CXX-2625] Bring our very own string_view (plus: Some iterators + ranges backports) (#1062)

* Bring our own std::string_view
* Shorthand for trailing return type, noexcept, and single-return-statement
* Some additional traits: rank<>
* New operators.hpp utilities
* We don't need to start_mongod for the simple compile tasks
* A new set of macros for concise diagnostic controls
* Reorganize macro definitions
* Exclude some files from Doxygen coverage
* Qualify bsoncxx::detail references