Adding a prototype attribute.

This attribute adds an additional way of declaring a prototype for a
sub, making sub foo($$) and sub foo : prototype($$) equivalent.  The
intent is to keep the functionality of prototypes while allowing other
modules to use the syntactic space it currently occupies for other
purposes.

The attribute is supported in attributes.xs to allow
attributes::->import to work, but if its defined inline via something
like sub foo : prototype($$) {}, it will not call out to the
attributes module.

For: RT #119251