update breadcrumbs for semantic markup

With this update you can also use the breadcrumb layout:
````HTML
<nav class="breadcrumbs">
  <a href="#">Parent</a>
  <a href="#">Child 1</a>
  <a href="#">Child 2</a>
 </nav>
````
with the negligible differences of $crumb-slash being slightly smaller due to inheriting the $crumb-font-size and losing the $crumb-link-decor because it would also apply to the $crumb-slash.