Slurpies are assumed to be optional

[schwern]

Method::Signatures supports required slurpies.

$ perl -MMethod::Signatures -wle 'method foo (@foo!) {} main->foo'
In call to main::foo(), missing required argument @foo at -e line 1.

The idea that slurpies are always optional is baked pretty deep into Function::Parameters and I'm having difficulty enforcing the requirement.

[mauke]

See #17. It's not that they're optional, it's that they consume a list and lists can be empty.