Function gen_store_impl 

fn gen_store_impl(fields: &[(&Ident, &Type)], packing: &Ident) -> TokenStream

Generate fn store() implementation.

For consecutive packable fields sharing a slot, accumulates changes in memory and writes once, avoiding redundant SLOAD + SSTORE pairs.

Zero-init behaviour (T4+)

Each packed slot group starts from U256::ZERO instead of a previous SLOAD, so any byte not written by a declared packed field is zeroed on every store. If a struct later on removes a trailing field, those formerly-occupied bytes will be cleared on the next write.