Replace 'string-to-vector with 'string-to-simple-array
Update 'digest-string to use 'string-to-simple-array
Add 'delete-all-triples
Make comments optional for narrative versions

Correcting misbehaviour introduced while trying to fix other bad
behaviour (is that yak-hair I can smell?)
If a narrative version is created without a comment, stop creating a
comment consisting of an empty string.
Add macro 'http-return-code to compactify tests

Remove lots of repeated boilerplate code by using a macro to generate
it.
This involves the additional of a test-utilities.lisp file to contain
the macro.
Export 'delete-role
Bugfix 'delete-role

Correctly call 'delete-all-triples when truncating the role's graph.

Also stop directly invoking '4store:delete-graph, as that introduces a
direct dependency on a supposedly swappable backend implementation.
Comment that out until 'delete-graph is implemented as a generic
function within ox.
Test creating a graph and adding it to a role