Renames (in rusql::mysql) error_checker -> ErrorCheckerConnection, StatementErrorChecker -> ErrorCheckerStatement
Renames throw_error to check_and_throw
Renames MySQLUseResult -> UseResult
Moves 'data'-functors (to get char* from vars) into field::buffer::, removes some unecessary templating
Moves get_field_length into field::length
Moves get_is_is_null to field::is_null