FStar backend usize literals support

[kaspar030]

We're hitting this:

error[HAX0001]: (FStar backend) something is not implemented yet.
                usize literals (fail if pattern maching, otherwise TODO)
   --> src/riot-rs-runqueue/src/runqueue.rs:143:17
    |
143 |                 0 => i.is_none(),
    |                 ^

[karthikbhargavan]

This is a known limitation. The issue is that we do not know the size of usize on the target platform.
So, it is better to cast the usize into u8or u16 or whatever in the pattern match:

match (x as u8) {.
  0 => ...
} ```

[W95Psp]

I'm closing that issue in favor of #464, and PR #602 changes the error to mention both Karthik's workaround and issue #464.

[kaspar030]

Thanks!