implement standard iostreams based on nng::msg

[EvanBalster]

This is a complete header-only implementation of the streambuf, and iostream interfaces for NNG messages.

I am not sure if supplementary features like this belong in NNGPP, but it's quite useful. As such, consider this PR tentative. I haven't yet conformed it to NNGPP's code style as I expect some discussion about whether it ought to be integrated.

For example:

nng::msg msg;
nng::omsgstream write(msg);
write << "POST /messages HTTP/1.1" << std::endl;
write << "Content-Type: text/plain" << std::endl;
write << std::endl;
write << "Hello, world" << std::endl;

This code relies on nng_msg_capacity and nng_msg_reserve functions, which I contributed to NNG only a few days ago. As such it requires the "tip" version of NNG to function.

In addition to reading and writing nng::msg like a file, a sub-range of the message may be specified using open_range or additional constructor arguments. This is intended for exposing sections of messages or omitting application-layer headers from I/O operations. The parameter scheme for specifying ranges is a recent addition and might need more thinking.

Unlike ordinary iostreams, msgbuf and msgstream are copyable, and multiple msgbuf may refer to the same message at the same time. No special provisions are made for concurrency.

I have performed basic consistency testing with this facility in my own library. If it's put into production, more detailed unit tests are probably necessary.

† Note: similar to most implementations of std::stringstream, these classes lack full support for the ios_base::app flag.

[EvanBalster]

I've fleshed the implementation out somewhat. I've gotten it working on osx/clang in addition to windows/msvc. It also now supports unusual character types such as std::byte and custom traits classes.