Update the rust toolchain to nightly-2024-04-03

[tautschnig]

Changes required due to:

• rust-lang/rust@3da115a93b Add Ord::cmp for primitives as a BinOp in MIR

Resolves: #3130

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT licenses.

[tautschnig]

The changes in this PR make Kani build with the 2024-04-03 toolchain, but compiletest then fails as we run into https://github.com/rust-lang/rust/blob/d009f60b55fe4527e7ddf122bc4520f351d7b9d4/compiler/stable_mir/src/mir/body.rs#L373 (i.e., an unimplemented! in stable_mir that was added as part of the PR that required those changes on our side in the first place). @celinval might be best placed to help resolve this?

[celinval]

Damn... this isn't good. This PR shouldn've been approved. This Fix definitely needs to be addressed on the rustc side.

[celinval]

BTW, the fix to StableMIR has been approved: rust-lang/rust#123655

[tautschnig]

BTW, the fix to StableMIR has been approved: rust-lang/rust#123655

Thank you @celinval for implementing this fix upstream! As this is now also merged we should be able to bump the toolchain tomorrow morning.

[tautschnig]

Closing in favour of #3144.