Rename multi-abi to something more explicit

[montyly]

multi-abi is a confusing name, and we have frequent questions about what it does. We could rename it to something like --all-contracts, which would be easier to understand.

This would require to update all our tutorials (including building-secure-contracts)

[ggrieco-tob]

Sure, we can include this in the next "major" release (with breaking changes).