Add no_std support for Substrate/WASM runtimes

[nol4lej]

Currently, light-poseidon cannot be used in Substrate runtimes or other WASM environments that don't have access to the standard library. This limits its adoption in the Polkadot/Substrate ecosystem and other blockchain platforms.

Add no_std support while maintaining full backward compatibility. I have already implemented and tested this in my fork at https://github.com/orbinum/light-poseidon-nostd

Changes Required

Cargo.toml:

• Add std feature flag (enabled by default)
• Configure all dependencies with default-features = false
• Make thiserror optional (only available with std)
• Add curve feature to ark-bn254 for Fr support in no_std

lib.rs:

• Add #![cfg_attr(not(feature = "std"), no_std)]
• Import extern crate alloc for no_std environments
• Use alloc::vec::Vec conditionally
• Create dual implementation of PoseidonError (with/without thiserror)

parameters/bn254_x5.rs:

• Import alloc::vec macro for no_std
• Replace std::convert with core::convert

Verification

All changes have been tested and verified:

# no_std build for WASM
cargo build --no-default-features --target wasm32-unknown-unknown ✓

# Standard build
cargo build ✓

# All tests pass
cargo test ✓ (35 tests)

Usage

# For no_std environments
[dependencies]
light-poseidon = { version = "0.4.0", default-features = false }

# For std environments (default, unchanged)
[dependencies]
light-poseidon = "0.4.0"

Benefits

• Enables use in Substrate runtimes and WASM targets
• Zero breaking changes (std enabled by default)
• All existing tests pass
• No performance impact

I'm ready to submit a PR with these changes.