Implement a converter of simple ShEx to SPARQL queries

[labra]

Some users are interested in converting simple ShEx shapes to SPARQL queries.

Of course, this conversion is not possible for complex ShEx, but it can be useful for simple cases and could be used, for example, for converting some Wikidata entity schemas to the skeleton of SPARQL queries that a user could later edit.

As an example, the following ShEx:

<Person> { 
 wdt:P31  [ wd:Q5 ]  ;
 wdt:P19  @<Place> * ;
}

could be converted to this SPARQL query:

SELECT * WHERE { 
 ?this wdt:P31 wd:Q5 .
 ?this wdt:P19 ?P19 .
}

[labra]

First implementation in PR #68 and added to release v0.0.13