chore: apply unreachable_pub lint to crates

[TomAFrench]

This PR rolls out the unreachable_pub lint over all the crates, fixing any issues raised. These are either items which have not been exported for quite some time or are very clearly internal implementation details.

I've used an inner attribute to activate the lint - my preference for this is to use the linting system in cargo.toml as it makes it easiest to enforce consistency over multiple crates but I didn't want to cause a split with the existing config.

[newpavlov]

I don't think I agree with this lint. It's pretty common in my experience to define a pub(crate) module and pub items in it with the implicit assumption that items will also be pub(crate). Same with pub(crate) struct and pub fn methods on it. There are also the sealed traits which use unreachability of pub items intentionally (I know people want to add a proper support for it, but the proposals do not have a clear timeline).

[newpavlov]

@tarcieri
WDYT? Should we be more strict with pubs? If yes, then we would need to gradually enable the lint in other repos as well.

[tarcieri]

I'm a fan of using pub(crate) when you mean pub(crate), since it will prevent accidental leakage, though sealed traits are a notable exception.

If it's just warn though, you can always do allow where you need exceptions

[newpavlov]

Ok, let's move forward with it then.

@TomAFrench
Could you create similar PRs later in other repos as well?

[newpavlov]

It's better to use either pub(crate) or pub(super) for both the function and the constants.

[TomAFrench]

Agreed, I've changed the remaining uses of super to basically just cover the architecture-specific compression functions as these are the only items which should definitely never be imported elsewhere in the crate.

[TomAFrench]

Should be good to go now

[newpavlov]

Thank you!