Fix cfgs

[kornelski]

Fixes #440

I've grouped combinations of features for each library as one-liners, so that they can be copy'n'pasted to where they're needed, without having to have a SAT solver in your head.

Unfortunately macros can't wrap struct fields, so cfg-if can't help here.

Verified with cargo-hack that all 239 combinations of feature flags compile okay:

cargo hack --feature-powerset --exclude-no-default-features --skip any_impl,any_zlib check

[Byron]

Thanks so much for your help and for sharing your insights earlier.

This should mean that all currently supported backends remain as they all offer certain benefits.

And when looking at the complexity of the cfg attributes, we'd probably also avoid picking up more backends 😅.