Fix Rust cosmetic pass removing trailing units

[R1kM]

Slight refinement of the nanopass removing trailing units in the Rust backend.
Consider the following code

let x: u32 = 0;
()

With the previous version, the nanopass would have rewritten this code into 0, leading to typechecking failures in Rust.

This PR tweaks the pass to only perform the rewriting when the previous expression has type unit.

[msprotz]

Alternatively, you could rewrite into let _x: u32 = 0; because you know for sure that it is unused. (If it doesn't already start with _).

I'm fine with either. Thanks!