Add unit keyword to module definitions

This is now required when defining a module whose scope is only within the
current unit.

Author: paultcochrane

lib/Perl6/Examples.pm

@@ -1,4 +1,4 @@
-module Perl6::Example;
+unit module Perl6::Example;
 
 use v6;
 

lib/Pod/Convenience.pm6

@@ -1,4 +1,4 @@
-module Pod::Convenience;
+unit module Pod::Convenience;
 
 sub pod-gist(Pod::Block $pod, $level = 0) is export {
     my $leading = ' ' x $level;

lib/Pod/Htmlify.pm6

@@ -1,4 +1,4 @@
-module Pod::Htmlify;
+unit module Pod::Htmlify;
 
 use URI::Escape;
 use Pod::To::HTML;