1.6: Adopt Turtle grammar for IRIs, CURIEs, literals

[cmungall]

From @balhoff:

• <http://purl.obolibrary.org/obo/UBERON_1234567> could be used anywhere that UBERON:1234567 can be used
• Strings are always quoted
• People may not like this everywhere, like the value for term names
• No need to guess what is a full IRI and what is an abbreviated IRI (CURIE)
• Solve current problems with IRI mangling
• Solve questions about language tags support

From @cmungall:

• I think making strings always quoted is a major breaking change; against
• I don't think the <> quoting is necessary

[althonos]

@cmungall : You don't need the <> quoting provided you change the rules for xref lists and qualifier lists (#148); otherwise, since , and = are valid IRI characters, every construction of the form [iri, iri, iri] or {iri="value"} is ambiguous.

Keeping IRIs non escaped could work if you require all identifier to be followed by at least a whitespace: then you would have to change the lists to be [ iri , iri , iri ] which looks a bit ugly but makes sure the IRI are terminated properly.