Reduce re-exports

[robin-nitrokey]

Based on the discussion in #155, this PR removes most re-exports of external types and all re-exports of internal types. The following re-exports are kept:

• heapless_bytes::Bytes (only from types): crucial part of the client API
• littlefs2::path::{Path, PathBuf}: crucial part of the client API
• littlefs2::fs::{DirEntry, Metadata}: also part of the client API, though less important
• cbor_smol::{cbor_deserialize, cbor_serialize_bytes}: kept for the time being, but should probably replaced with more specific (de-)serialization methods for the data returned or expected by the API at some point

Depends on:

• https://github.com/trussed-dev/trussed/pull/160