New PR: yet another refactoring of the semantics

Dear all,

recently I was increasingly bothered with some aspects of the RDF-star 
semantics:

* the use of hidden IRIs is clearly not ideal, all the more that "they 
can not hide from SPARQL" (https://github.com/w3c/rdf-star/issues/101)

* the SPARQL-star execution semantics was not aligned with the MT semantics

This PR is an attempt to solve these two problems:

     https://github.com/w3c/rdf-star/pull/162

which I propose we discuss during our next call.

The main change with the previous version, apart from not using hidden 
IRIs, is that unstar(G) is no longer equivalent to G (but they are 
equisatisfiable).

Working on this PR, I realize that I was actually coming back (or very 
close) to a solution proposed by Peter Patel-Schneider quite some time ago

https://lists.w3.org/Archives/Public/public-rdf-star/2020Nov/0044.html

Peter, I guess I owe you an apology for not seeing the value in this 
proposal earlier!...

   best