make lexical prefix a counter

The boolean state is was using right now was broken as soon
as we have more than one level of prefix.

    prefix '/foo' => sub {
        # local_prefix is now true
        prefix '/bar' => sub {
            $ still on, all's good
        };
        # oh crap, state set to false, everything goes to hell!
        prefix '/baz' => sub {
            # all is broken here :-(
        };
    };

Fixes #842
changelog
Merge branch 'embedded-prefixes' into devel