Add an option to set default traits lexically.

[schwern]

Have an option to declare default traits to be applied to every parameter in this scope.

# Turn on coercion and make all parameters read only aliases.
use Method::Signatures { traits => { "coerce", "ro", "alias" } };

Use cases include turning on type coercion or making all parameters read-only.

Care must be taken because some traits are the opposites of the other. In the above example, you'd want "is rw" to override the default read only trait. Or you'd want "is copy" to override the default alias trait.