refactor: Introduce Storable trait

[matheus23]

Main use case is allowing us to use Link<UnixFsFile> as a type in PublicFile::userland.
Other use case is allowing us to nest the WNFS Hamt: Node<String, Node<String, String>> is now possible (previously it required K: Serialize and V: Serialize (and we don't generally want to make both K: AsyncSerialize and V: AsyncSerialize.

Also:

• Remove AsyncSerializable
• Remove RemembersCid (both of these are now handled by Storable)
• Remove TryFrom<Ipld> instances that were used for deserialization of some types
• Remove BlockStore::put_async_serializable function (you can just use Storable::store instead)
• Introduce NodeSerializable and HamtSerializable for wnfs-hamt to follow the pattern used in wnfs.

TODO

• Write documentation for Storable trait & functions
• Look at wnfs-wasm

[codecov]

Codecov Report

Merging #378 (d95840d) into main (2b67800) will increase coverage by 0.75%.
The diff coverage is 40.45%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #378      +/-   ##
==========================================
+ Coverage   53.86%   54.62%   +0.75%     
==========================================
  Files          55       56       +1     
  Lines        3891     3879      -12     
  Branches      954      933      -21     
==========================================
+ Hits         2096     2119      +23     
+ Misses       1182     1161      -21     
+ Partials      613      599      -14     

Files	Coverage Δ
wnfs-common/src/blockstore.rs	57.14% <ø> (+5.62%)	⬆️
wnfs-common/src/encoding.rs	62.50% <ø> (ø)
wnfs-hamt/src/hamt.rs	68.75% <ø> (+27.72%)	⬆️
wnfs-nameaccumulator/src/name.rs	82.53% <ø> (ø)
wnfs/examples/private.rs	0.00% <ø> (ø)
wnfs/examples/tiered_blockstores.rs	0.00% <ø> (ø)
wnfs/examples/write_proofs.rs	0.00% <ø> (ø)
wnfs/src/lib.rs	90.00% <ø> (ø)
wnfs/src/private/directory.rs	68.92% <ø> (ø)
wnfs/src/root_tree.rs	63.23% <ø> (ø)
... and 12 more

... and 2 files with indirect coverage changes