Add relativizer

[lo48576]

relativize_against() or similar functionalities would be useful for servers or static site generators.

[lo48576]

What is the result of relativizing foo:////bar against foo://?

• //bar would be valid as a path, but not interpreted as expected if it is a relative IRI reference.
  • In this case, resolving //bar against foo:// is foo://bar.
• .///bar would be valid both as a path and as a relative IRI reference, but is it intended by the user?

Would making the algorithm configurable be a solution?

[lo48576]

Relativizing foo:///bar:baz against foo:/// would have similar problem.
What is expected, bar:baz (interpretable as scheme=bar and path=baz), or ./bar:baz?

[lo48576]

The two results would have different types (a path and a relative IRI reference), so maybe the feature should be provided by different names, for example relativize_as_path_against() and relativize_as_iri_ref_against().