RISC-V: Use generic tree structure for MerkleProof

Part of RV-233

What

Introduce a Tree<A> enum type and change MerkleProof = Tree<MerkleProofLeaf>.

Why

Distinguishing between intermediary nodes and leaves at the type level in a payload-agnostic makes writing the deserialisation more straight forward and more type-safe.

How

TLDR, introduced the type:

pub enum Tree<A> {
    Node(Vec<Self>),
    Leaf(A),
}

pub type MerkleProof = Tree<MerkleProofLeaf>;

pub enum MerkleProofLeaf {
    Blind(Hash),
    Read(Vec<u8>),
}

Manually Testing

make -C src/riscv all
# In particular
(cd src/riscv && cargo test proof)

Benchmarking

Interpreter performance not impacted

Tasks for the Author

• Link all Linear issues related to this MR using magic words (e.g. part of, relates to, closes).
• Eliminate dead code and other spurious artefacts introduced in your changes.
• Document new public functions, methods and types.
• Make sure the documentation for updated functions, methods, and types is correct.
• Add tests for bugs that have been fixed.
• Put in reasonable effort to ensure that CI will pass.
• Benchmark performance and populate the table above if needed.
• Write commit messages to reflect the changes they're about.
• Self-review your changes to ensure they are high-quality.
• Complete all of the above before assigning this MR to reviewers.