WASM: Remove [Chunked_bytes_vector.Buffer]

Context

This indirection makes the parse more complex for no specific reason, and allows reusing !5881 (merged) to merkleize the parser's state. This commit also provides a safe implementation of to_string and to_bytes.

Manually testing the MR

make test-webassembly to check the changes.

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