refactor: create crypto submodules for each precompile

[jdh847]

Summary

Closes #2782

Creates a dedicated crypto.rs submodule for each precompile that has mixed runner + cryptographic code, isolating pure cryptographic implementations from EVM-specific precompile logic (gas accounting, input parsing, output formatting).

Changes per precompile:

• secp256k1: Moved ecrecover_bytes and cfg_if! backend selection (k256 vs bitcoin_secp256k1) into secp256k1/crypto.rs
• secp256r1: Moved verify_signature and feature-gated implementations (p256 vs aws-lc-rs) into secp256r1/crypto.rs
• blake2: Moved the entire algo module (SIGMA, IV, G function, compress, round) and AVX2 optimized variant into blake2/crypto.rs
• modexp: Moved modexp function (GMP + aurora-engine-modexp fallback) into modexp/crypto.rs

Not changed (already separated):

• bn254: Already has arkworks.rs/substrate.rs as crypto_backend
• bls12_381: Already has arkworks.rs/blst.rs as crypto_backend
• kzg_point_evaluation: Already has arkworks.rs/blst.rs as crypto_backend
• hash: Crypto lives entirely in the Crypto trait default implementations (thin wrappers around sha2/ripemd crates)

Design decisions:

• No API changes — all existing paths (e.g., crate::secp256k1::ecrecover_bytes) still work via re-exports
• One commit per precompile for easy review
• Crypto trait in interface.rs unchanged — default implementations still delegate to the same functions
• Backward-compatible re-export pub use crypto::algo in blake2 preserves the blake2::algo::compress path

Test plan

• cargo check -p revm-precompile — compiles clean
• cargo clippy -p revm-precompile --all-targets --all-features — no warnings
• cargo test -p revm-precompile — 52 tests pass, 0 failures