RFC: Stop testing feature combinations exhaustively

[lnicola]

It looks like the tests take about 12 minutes or so, but the CI workflow uses cargo-all-features, which checks all combinations of features. We currently have three of them, which means that the tests (probably) run 8 times.

Rust features are supposed to be additive. I realize this isn't always the case (e.g. the PROJ network feature might influence the results), but there's not that much place for strange interactions between them:

• use-proj only re-exports some proj types AFAICT
• proj-network only enables a proj feature; this could presumably influence some (doc-)tests though
• serde only enables the serde integration

So I think it would be relatively safe to only run cargo check --no-default-features and cargo test --all-features, and maybe trim down the tests where proj-network could have an impact.

[urschrei]

I think that we probably don't have to test proj-network since any failure would occur and be evident upstream – we already test that enabling the network functionality changes the results in the proj crate, so barring a failure mode in which we accidentally deploy a proj crate version despite broken functionality and tests I think we can just use use-proj here?

[lnicola]

I agree, testing with proj-network only introduces new failure modes for us: getting different results because of the extra grids and not working because of transient network issues.