Ditch URI as namespace identifier

URI is a bit baroque for our needs as namespace representation. It is not as clunky as java.util.Date was for revision-date, but there are still issues.

Most notably we routinely need to convert between String-based Java namespace and URI, hence we end up adding caches, which is suboptimal.

Bite the bullet and define a yang.common.Namespace, which will just wrap a String and validate it.