Rename `object_safe` module

[sosthene-nitrokey]

"Object safety" is being renamed to "Dyn compatibility" in Rust: rust-lang/rust#130852

It might make sense to adapt it here too in the next breaking release (or maybe earlier with a pub use object_safe as dyn_compatible)

[robin-nitrokey]

In littlefs2-core, we already dropped the module entirely. In littlefs2, we only add DynStorage and FilesystemCallback so we could just remove the re-exports and move these two types to the top level.