Add decode to Message Pack (compact variant)

[leighmcculloch]

What

Add decode XDR to Message Pack's compact form.

Why

I wrote this decode to Message Pack to experiment with encoding contract specs in Message Pack, that are currently encoded as XDR.

To install the CLI in this PR, run:

cargo install --locked --git https://github.com/stellar/rs-stellar-xdr --branch decode-to-messagepack --features cli

To try out converting some XDR to Message Pack:

cat spec.xdr | stellar-xdr decode --type ScSpecEntry --input stream --output message-pack

Related discussion is here:

• https://discord.com/channels/897514728459468821/1283687871491997716/1284006942410539069