Update to the latest Rust toolchain (2023-11-06).

The relevant changes are:

• rustc_span cleanups rust-lang/rust#117507: this required changing the import of Span from rustc_span::source_map::Span to rustc_span::Span.
• Support enum variants in offset_of! rust-lang/rust#114208: this changed the data field for the OffsetOf variant of NullOp from List<FieldIdx> to List<(VariantIdx, FieldIdx)>, which required updating the relevant code in rvalue.rs.
• Clean up unchecked_math, separate out unchecked_shifts rust-lang/rust#115626: the unchecked shift operators have been separated from the unchecked_math feature, so this required changing the feature annotation in tests/ui/should-panic-attribute/unexpected-failures/test.rs
• Some rustc change (not sure which one) result in a line in tests/coverage/unreachable/variant/main.rs getting optimized out. To maintain what this test is testing, I changed the match to make it a bit less-prone to optimization.
• A change in cargo (Add new packages to [workspace.members] automatically rust-lang/cargo#12779) resulted in an update to Kani's workspace Cargo.toml when cargo add is executed inside tests/script-based-pre/build-cache-bin. This is apparently intended behavior, so I had to make the exclude in the Cargo.toml more specific to make sure this doesn't happen (I tried using a glob, but that didn't work, apparently because of cargo should support glob syntax on workspace excludes rust-lang/cargo#6009.

Resolves #2848

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