feat: `backend-native-contracts`

[mkovaxx]

Feature: Rustc Contracts Backend

Implement an anodized backend that targets the upstream nightly rustc-contracts experiment.

• Expansion strategy:
  • requires clauses map 1:1 to #[core::contracts::requires(...)].
  • ensures clauses map 1:1 to #[core::contracts::ensures(...)].
  • maintains clauses emit both attributes so invariants remain enforced on entry and exit.
• Unsupported today: captures have no equivalent in the nightly design, so the backend must fail with an error when encountering that.
• Activation: exposed via a backend-native-contracts feature that only builds on nightly toolchains and switches code generation to emit the attributes of the nightly contracts experiment.
• Testing: sample crate compiled with -Zcontract-checks to prove parity with the language-team experiment.

[mkovaxx]

After some research, the idea to target another spec/contract systems using a backend for the proc macro seems infeasible in general.

There's likely not enough context available in a proc macro, e.g. lack of type information.

I've decided not to pursue this idea any further, in favor of putting more effort into the import/export mechanism, where all necessary information can be obtained for complex transformations.