Embed shapes with anonymous nodes

[labra]

A shape like the following:

:S {
 & _:genid0
}
_:genid0 {
 :p  xsd:string
}

is equivalent to

:S {
 :p xsd:string
}

Implement a linter algorithm to simplify these shapes.

This is important to clean the results generated by the SHACL->ShEx converter (#113)