fix: rustc-hash conditional compilation in reth-ethereum-forks

[reject-i]

Running cargo test -p reth-primitives-traits --lib --no-default-features was failing with unresolved import rustc_hash error because rustc_hash::FxHashMap was imported with only std feature but rustc-hash dependency requires rustc-hash feature.

Fixed conditional compilation to require both std and rustc-hash features for FxHashMap import and added std::HashMap fallback when rustc-hash is disabled.

[reject-i]

@mattsse removed the optional rustc-hash feature so the crate now always depends on it and switches to FxHashMap whenever std is on
cargo test -p reth-primitives-traits --lib --no-default-features gets past the previous rustc_hash import error and only hits the long-standing modular_bitfield issue (that crate is still optional unless reth-codec is enabled), which predates this pr

would you like me to fix that here as well, or should I spin it out into a follow-up pr ?