Type-based secret independence

[karthikbhargavan]

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This is an initial PR to include type-based secret independence in Libcrux.

The new crate secrets introduces types and functions for secret integers. A future PR will provide a more comprehensive crate with secure erasure and constant-time ops. To enable secret independence checking, it provides the feature check-secret-independence. If this flag is not set, the crate implements all integers transparently, with (hopefully) no performance cost.

The secrets crate has been tested in libcrux-ml-kem. Specifically, this PR also adapts the code within vector/portable to use secret integers. To check secret independence, call cargo check --features "check-secret-independence"