Upgrade to syn 2.0

[ruifengx]

Upgrade the dependency syn to 2.0.

The major changes in syn 2.0 related to this crate:

• structure of Attribute changes, and Attribute::parse_meta is replaced by Attribute::parse_nested_meta and Attribute::parse_args[_with]
• Keywords in meta path positions cause parsing errors dtolnay/syn#1458

The first is trivial to handle. The parentheses of attributes are now dealt with by Attribute::parse_args, so I removed the parsing for parentheses in CallMethodAttribute, IntoAttribute, etc. The second one is more tricky, because await is now considered a keyword and #[await(true)] is rejected by Attribute::parse_outer. I had to duplicate the relevant parsing functions from syn and make a tolerant variation (see tolerant_outer_attributes in lib.rs).

All the tests pass, and there should not be any breaking changes.

[Kobzol]

Hi, thanks a lot! I was also thinking about this, but I was worried that the switch could break something so I didn't rush into it 😅 Also thanks for some of the typo/wrong error message fixes.