Expose error_category's internal unique id

For https://github.com/ned14/status-code/blob/master/include/boost_error_code.hpp#L109 I'd like to be able to access `error_category`'s internal unique id.

For https://github.com/ned14/status-code/blob/master/include/boost_error_code.hpp#L101 I'd like the ability to get some singleton `error_category&` for a specific `error_category` unique id. Then unique category id's can always be turned into an error category singleton, which makes exposing the unique id more widely useful.