This PR fixes #12

It adds a facility to generate a split wrap key (and associated authentication key) without generating other keys:

[user@localhost release]$ ./yubihsm-setup genwrapkey --help
yubihsm-setup-genwrapkey
Create a new split wrapkey with associcated auth key

USAGE:
    yubihsm-setup genwrapkey [OPTIONS]

FLAGS:
    -h, --help       Prints help information
    -V, --version    Prints version information

OPTIONS:
    -l, --label-prefix <label-prefix>    Label prefix for wrap key and auth key [default: ]

The optional --label-prefix parameter allows you to specify a prefix for the key labels, so they become:

• prefix Application auth key
• prefix Wrap key

This is my first delve into Rust (so be gentle on my change!!) - I built this locally with Rust 1.61 - it would not build with later versions (I think some deprecations have become errors)