Add compare sub-command to cli

[leighmcculloch]

What

Add compare sub-command to cli that compares XDR values returning a -1 for less-than, 0 for equal, and 1 for greater-than.

Example

An ScVal symbol with value "Ed25519" is less than "Policy":

$ stellar-xdr compare \
    --type ScVal \
    <(echo '{"symbol":"Ed25519"}' | stellar-xdr encode --type ScVal) \
    <(echo '{"symbol":"Policy"}' | stellar-xdr encode --type ScVal)
-1

Why

Stellar SDKs are having to write logic for sorting ScVal's specifically, and this tool will help with testing and verifying SDK implementations since the Rust XDR lib can already do the ordering.