* lang: implement `Key` for `Pubkey` again, so `associated_token::*` constraints can use pubkey targets again ([#1601](https://github.com/project-serum/anchor/pull/1601)).
* lang: adjust error code so `#[error_code]` works with just importing `anchor_lang::error_code` ([#1610](https://github.com/project-serum/anchor/pull/1610)).