Add 2 admin commands to compare 2 package versions

[mbarbin]

Add 2 new commands opam admin {assert-}compare-versions to compare package versions for sanity checks.

1. compare-versions - User interactive.

In this command, we provide two versions, and opam outputs a user-friendly expression with the result of the comparison, spelled out. For example:

$ opam admin compare-versions 0.0.9 0.0.10
 0.0.9 < 0.0.10

The command exits 0 regardless of the result of the comparison.

2. assert-compare-versions - For use in scripts, if needed.

This command is silent, and requires an operator {lt,le,eq}. The result of the command will be encoded in the exit code, to be used in bash conditionals:

exit 0: the comparison holds
exit 1: it doesn't

For example:

$ opam admin assert-compare-versions 0.0.9 lt 0.0.10
[0]

$ opam admin assert-compare-versions 0.2.2 le 0.2.0
[1]

This PR is an alternative to #6124 trying to account for the various feedback received.