simplify internal visibility by replacing conditional flags with pub(crate) prefixes

[sebastiantia]

we can define the default visibility of delta internals by prefixing the struct/fn/... with pub(crate) instead of #[cfg_attr(not(feature = "developer-visibility"), visibility::make(pub(crate)))]. This simplifies the codebase.

Examples:

delta-kernel-rs/kernel/src/snapshot.rs

Line 145 in 06d8dbb

#[cfg_attr(not(feature = "developer-visibility"), visibility::make(pub(crate)))]

delta-kernel-rs/kernel/src/actions/visitors.rs

Line 21 in 06d8dbb

#[cfg_attr(not(feature = "developer-visibility"), visibility::make(pub(crate)))]

and many more...