Explore mutable/immutable classes

[stevan]

Also explore the idea of a mutable and immutable role/class, possibly this is returned from an API call such as ...

BEGIN {
    my $mutable = mop::util::get_meta( $package_name ); 
}

my $immutable = mop::util::get_meta( $package_name ); 

mop::util::open_class( $package_name => sub {
    my $meta = @_; # mutable instance ...
});

[stevan]

Did this in the role-immutable-mutable-split branch in c80131f0bf2400552786aabe5f4fec8779379ba5, not sure I like it.