Upgrade to syn 2.x

[Shnatsel]

The fax crate has been at last integrated into tiff crate (and by extension image crate) in image-rs/image-tiff#229

However, it is not yet enabled by default. The tiff maintainers have raised the dependence on syn 1.x as a blocker for enabling fax decoding by default: image-rs/image-tiff#294

It would be great to get the fax crate migrated to syn 2.x so that fax decoding could be enabled in tiff and image by default.

[s3bk]

I have updated syn to 2.0. fax version 0.2.6

[Shnatsel]

Oh, that was quick! Thank you!