Implement Token! macro for syntax kinds, a-la syn

[matklad]

Syn has a lovely Token! macro for defining tokens without naming them: Token![,]:

https://docs.rs/syn/0.15.33/syn/macro.Token.html

We should add a similar macro for our SyntaxKind, to make code like this more readable:

https://github.com/rust-analyzer/rust-analyzer/blob/master/crates/ra_ide_api/src/extend_selection.rs#L137-L138

[matklad]

Though, I am not sure if macros are allowed in patterns. If they aren't, then probably we should stick with the current approach.

[edwin0cheng]

Macro is allowed in pattern :) See #1220

[pasa]

I have never created rust macro. I think it is a good day to start.

[pasa]

Hi. I've made PR with T! macro apply for whole project. I hope it will be helpful.
In addition I figured out that T![(] and T![)] are not working so I've changed them to T!['('] and T![')'].

[lnicola]

Can this be closed?