Clean-up guest builds

[capossele]

As the [target.'cfg(not(target_os = "zkvm"))'.dependencies] does not seem to work for some reason, this PR exposes the feature host (enabled by default) to prevent unnecessary deps ending up in the guest builds

[Wollac]

Looking at the Cargo.lock it seems to work, but I do not understand the details:

• The TOML still contains [target.'cfg(not(target_os = "zkvm"))'.dependencies]. So this is still needed, but simply adding the optional will do the trick?
• Building the lib with --no-default-features now fails because the target OS is not zkvm, but all those dependencies are not included. This also causes problems with the rust-analyzer. Is there a way to fix this? One alternative I could think of is to remove all #[cfg(not(target_os = "zkvm"))] and manually enable/disable the host feature. However, I am not sure if this is a good idea.

[capossele]

Yeah I'm not sure why [target.'cfg(not(target_os = "zkvm"))'.dependencies] does not really work as expected. It can be removed though if properly switching as you suggested from #[cfg(not(target_os = "zkvm"))] to #[cfg(feature = "host"))].
I've pushed a commit doing that.

Overall this is just a suggestion, maybe a bit hacky, so no harm in closing it if you guys are not convinced. Obviously would be better to understand why [target.'cfg(not(target_os = "zkvm"))'.dependencies] does not properly work

[capossele]

Actually I did not feel comfortable at removing the #[cfg(not(target_os = "zkvm"))]. So I ended up having them both

[Wollac]

It seems that [target.'cfg(not(target_os = "zkvm"))'.dependencies] actually works as expected, but the disabled dependencies show up in the Cargo.lock of the guest. However, they are resolved correctly. See: rust-lang/cargo#10801