Add Experimental Message & Socket API

[rbx]

• Add fair::mq::Msg class that is move-only and can contain multiple buffers. Resolves #101.
• Add corresponding transfer APIs that take ownership over the Msg and introduce new return value. Resolves #85. The return value fair::mq::TransferResult consists of:

enum class TransferCode : int
{
    success = 0,
    error = -1,
    timeout = -2,
    interrupted = -3
};

struct TransferResult
{
    size_t nbytes; // number of bytes transferred
    TransferCode code; // see above class
    std::optional<Msg> msg; // optional Msg, e.g. if transfer failed or timed out
};