Lib_scoru_wasm: Tree encoders-decoders

Context

Work based on this branch from @ole.kruger.

This MR introduces a new module Tree_encoding_decoding that can be used for constructing encoders/decoders similar to Data_encoding.

The plan is to:

• Address #3081 (closed) by replace the Wasm_decodings module to one that uses the Tree_encoding_decoding to also produce encoders.
• Use for #3083 (closed) to encode/decode configuration.

The Tree_encoding_decoding can be compatible with Thunk by using the Thunk.custom function, see !5654 (merged).

Manually testing the MR

dune exec src/lib_scoru_wasm/test/test_scoru_wasm.exe -- test "$Encodings^"

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR