Fix breaking change: Do not enforce discriminant to be a literal + don't expect usize.

[gui1117]

Fix breaking change: #687
Fix forcing usize for variant expression introduced in: #653

Currently testing with polkadot-sdk: paritytech/polkadot-sdk#7417

Maybe we can have a new attribute: allow duplicate index.

[serban300]

LGTM !

[gui1117]

clippy fails with:

Run cargo +stable clippy --locked -- -Dwarnings
  cargo +stable clippy --locked -- -Dwarnings
  cargo +stable clippy --locked -p parity-scale-codec-derive -- -Dwarnings
  cargo +stable clippy --locked --test clippy -- -Dwarnings
  shell: sh -e {0}
  env:
    IMAGE: paritytech/ci-unified:bullseye-1.81.0
    CACHE_ON_FAILURE: false
    CARGO_INCREMENTAL: 0
    Updating crates.io index
error: the lock file /__w/parity-scale-codec/parity-scale-codec/Cargo.lock needs to be updated but --locked was passed to prevent this
If you want to try to generate the lock file without accessing the network, remove the --locked flag and use --offline instead.

but it works for me locally, I will merge and try to trigger ci in another PR