Escaping for URL-as-ID is inconsistent with IRIs

[althonos]

The way special characters are escaped is not consistent with IRIs, so it's hard to treat them equivalently: the URL-as-ID production rule uses backslash escaped characters, but escaping should use the URL/URI/IRI percent-encoding instead to be compatible with IRI syntax.

[althonos]

This could be fixed by requiring the URL-as-ID to be following the URL production rule from the Uniform Resource Locator grammar, or by actually using actual IRIs as identifiers, similarly to the OWL2 Manchester syntax.