add macro for enum types

[laroque]

At various times it is useful to have a conversion between an enum type and a string (for example, a user-written config file will have the string name which we want to unpack to an enum value). A nice pattern looks like:

file.hh

    enum class op_t:uint32_t {
            set = 0,
            get = 1,
            config = 6, // deprecated as of v2.0.0
            send = 7,
            run = 8,
            cmd = 9,
            unknown = UINT32_MAX
    };

file.cc

    DRIPLINE_API std::string to_string( op_t an_op )
    {
        switch (an_op) {
            case op_t::set: return "set";
            case op_t::get: return "get";
            case op_t::config: return "config";//config is deprecated
            case op_t::send: return "send";
            case op_t::run: return "run";
            case op_t::cmd: return "cmd";
            case op_t::unknown: return "unknown";
            default: throw dripline_error() << "op_t value <" << an_op << "> not recognized";
        }
        //TODO explicitly throw something here?
    }
    DRIPLINE_API op_t to_op_t( std::string an_op_str )
    {
        if ( an_op_str == to_string( op_t::set ) ) return op_t::set;
        if ( an_op_str == to_string( op_t::get ) ) return op_t::get;
        if ( an_op_str == to_string( op_t::config ) ) return op_t::config;
        if ( an_op_str == to_string( op_t::send ) ) return op_t::send;
        if ( an_op_str == to_string( op_t::run ) ) return op_t::run;
        if ( an_op_str == to_string( op_t::cmd ) ) return op_t::cmd;
        if ( an_op_str == to_string( op_t::unknown ) ) return op_t::unknown;
        throw dripline_error() << "unable to map <" << an_op_str << "> to an op_t value";
        //TODO explicitly throw something here?
    }

It would be nice if this were all able to be done in a macro, the amount of logic is large (dealing with entries with and without specified value assignments, variable number of entries, etc.). Also, we have to decide if we are either okay with having the functions in the header, or if there is a way for the macro to also edit the source file, or some other pattern that isn't obvious to me yet.