Replace assume with assert_unchecked (Rust 1.81)

[kkysen]

Rust 1.81 stabilized std::hint::assert_unchecked, which is what we want our assume to be. Our assume is a slightly different implementation (that just calls unreachable_unchecked in the false case), and assert_unchecked is implemented more directly with regards to LLVM IR generation. They are largely the same, but assert_unchecked may be optimized slightly better, and Rust is standardizing around this new interface, so we should switch to it.