Optimize

Right now, the "match statement" generated by this macro is of the form:
```rust
match thread_rng().gen_range(3) {
x if x == 0 => { /* block 0 */ }
x if x == 0 + 1 => { /* block 1 */ }
x if x == 0 + 1 + 1 => { /* block 2 */ }
_ => panic!(),
}
```
(Thanks to [this thread](https://users.rust-lang.org/t/how-to-generate-in-macro/56774/6) for that neat trick!) A really naïve compiler will fail to simplify those patterns, and thus will walk the match arms from top to bottom.

It would be nicer to generate something direct
```rust
match thread_rng().gen_range(3) {
0 => { /* block 0 */ }
1 => { /* block 1 */ }
2 => { /* block 2 */ }
_ => panic!(),
}
```
but as far as I know Rust declarative macros can't do this. The macros can't do the arithmetic to simplify the patterns, and the pattern matcher won't take constant expressions, so gridlock.

No file or test is named. Start by locating the macro that generates the match statement and inspect its generated Rust output; compare the current guard-based arms with the requested literal-arm form. Done means the generated match uses direct literal patterns without regressing the macro's existing behavior.

Written by the indexing model from the issue text.