Function is_gruu 

pub fn is_gruu(uri: &Uri) -> bool

Whether this URI is a GRUU (§4.5).

Presence is the entire test. A valueless gr is as much a GRUU as one carrying a value — §7 makes the value optional, and it is the form a registrar commonly mints temporary GRUUs in, where the whole URI is opaque and there is nothing for a value to add.