Fix compilation on the latest nightly

[Shatur]

The feature const_generics was spitted to adt_const_params and generic_const_exprs, see more infor here. So in this PR I replaced const_generics with adt_const_params. But the compiler complains that the feature is incomplete, so I added incomplete_features to allow.

Also I removed extended_key_value_attributes since it was stabilized in 1.54.0.

Fixes #16.

[ErnWong]

Thanks!

[Shatur]

Fixed other warnings that caused issues with CI.

[ErnWong]

Cheers!

[Shatur]

@ErnWong looks like something wrong with your token for test coverage?

[ErnWong]

Yeah. If I remember correctly, GitHub Actions doesn't share my test coverage token secret to the test-coverage PR check for forks (I might be wrong - I'll need to double check someday). I also remember reading somewhere that a token isn't actually required for public repos, but last time I tried it, I seemed to had difficulty getting it working without a token.