Add RelURI, AbsURI

UriAuth is under Maybe - as I understood due to wrap relative/absolute URIs under one type. If I have abs. URI I will propagate those Maybe anywhere. Better may be to distinguish abs. and rel. URIs - with different types. When I parse rel. URI (reference) - no any auth. In this case combining URIs will be checked on type level: rel+rel, abs+rel, but no abs+abs.