Skip simple strings in N3Quoter::escapeLiteral
This makes the function about 50% faster on the first 5,000 Wikidata
entities. That makes the truthy dump about 7% faster in general.
This might have more or less effect on the entire truthy dump, but
I'm not willing to benchmark this in detail.
Bug: T176844
Change-Id: I7141a58a022a98373c1390ee4e336fb9ee54f6c2