gen-rust: three type defects found by t27o (char literals, read-only []usize, []const u8 as &'static str) #8971

Found while building t27o (see #8966, PR #8970). The spec works around each defect, but the generator should be fixed in .t27 / t27c:

1. A t27 char literal ('a', a u8) is emitted as a Rust char, so arithmetic or comparison with u8 fails to compile.
2. A read-only []usize parameter is emitted as Vec<usize>, so callers that pass a slice fail.
3. []const u8 is typed as &'static str unless the file also contains a slice expression, so the same parameter type compiles differently depending on unrelated code in the file.

Related: the Zig backend drops parentheses in !(...) and -(...) (see #6549), and the optimizer hoists repeated subexpressions above their locals' declarations (see #8197).

Acceptance: a gen-rust conformance case for each of the three, compiled with rustc on the lab, with the generated Rust using u8, a slice and &[u8] respectively.