JSON: use enums instead of symbols

[asterite]

This changes JSON::Lexer and JSON::PullParser to expose enums instead of symbols for token kinds and pull parser state kinds. Symbols work but they are not type-safe. Plus enums gives enumerate (duh) all possible states.

The PullParser has already proven to be very useful but moving forward I wouldn't like to keep the Symbols interface so it's better now or never.

This is a breaking change:

• JSON::Lexer::Token#type : Symbol is replaced by JSON::Lexer::Token#kind : JSON::Lexer::Token::Kind
• JSON::PullParser#kind : Symbol is replaced by JSON::PullParser#kind : JSON::PullParser::Kind

To make it easy to upgrade I kept JSON::Lexer::Token#type : Symbol as deprecated and also made JSON::PullParser::Kind be comparable to symbols, with a deprecation notice. That way we don't actually break anything but if you run the compiler with --warnings=all then you can fix things at your time.

Once merged, in a subsequent PR I'll try to document all of these types, including the ones in the YAML module.

I also added JSON::Builder#next_is_object_key? because I think it's useful and I at least needed it for optimizing one thing in https://github.com/Blacksmoke16/oq (though I didn't send a PR there yet).

[j8r]

Even the Token class could be avoided, see https://github.com/j8r/con/blob/master/src/lexer.cr .
I was planning to do it, but never get enough time to.

[RX14]

I'm not so sure about the symbol compatibility using ==, it seems very limited in the code which it will not cause a breaking change in. But I might be wrong.

[asterite]

You mean it might cause a breaking change after all?

[RX14]

Maybe? The only way to tell would be to find a corpus of projects which use these classes

[asterite]

Even if it's not completely backwards compatibile I'd move forward with this. Upgrading it should be relatively straight forward.