Improve consistency and maintainability.

[markcol]

Changes to make the codebase more consistent:

• Changed StringType::empty() to StringType::new()
• Changed name of 'meta' field in Items to 'trivia'
• Changed Item::meta() to Item::trivia()
• Changed Item::meta_mut() to Item::trivia_mut()
• Add Parser::inc_n() to eliminate multiple Parser::inc() calls

Make working with Key and StringType delimiters easier:

• Add function KeyType::delimiter() and tests
• Add function StringType::delimiter(s: &StringType) and tests

[LeopoldArkham]

Thanks!