turtle output from various generators and dumpers should use canonical prefixes

rather than this:

```turtle
@prefix ns1: <http://www.w3.org/ns/shacl#> .
```

it should use the declared shacl prefix.

This affects a subset of the dumpers and also the rdflib dumper