feat: add various parallel and cryptographic examples

[zouguangxian]

Summary

This PR adds several new examples to the project, focusing on parallel algorithms and cryptographic operations.

New Examples

• parallel-keccak: Parallel implementation of Keccak hash function.
• batched-signatures: Example of processing signatures in batches.
• merkle-tree: Implementation of Merkle tree operations.
• parallel-mergesort: Parallelized merge sort algorithm.
• matrix-multiply: Parallel matrix multiplication.
• prefix-sum: Parallel prefix sum (scan) operation.
• fft: Fast Fourier Transform implementation.
• wavelet-transform: Discrete Wavelet Transform example.
• graph-bfs: Breadth-First Search on graphs.
• block-compression: Example of block-based data compression.
• polynomial-eval: Parallel polynomial evaluation.

Test plan

• Verified that each example can be compiled and added as a workspace member.
• Each example includes its own main.rs and lib.rs for testing and usage.