Resolve manual_let_else clippy lints

    error: this could be rewritten as `let...else`
      --> generate/src/main.rs:50:13
       |
    50 | /             let new = match u8::try_from(chunkmap.len()) {
    51 | |                 Ok(byte) => byte,
    52 | |                 Err(_) => panic!("exceeded 256 unique chunks"),
    53 | |             };
       | |______________^
       |
       = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#manual_let_else
       = note: `-D clippy::manual-let-else` implied by `-D clippy::pedantic`
    help: consider writing
       |
    50 ~             let Ok(byte) = u8::try_from(chunkmap.len()) else {
    51 +         $crate::rt::begin_panic($msg)
    52 +     };
       |

generate/src/main.rs

@@ -47,9 +47,8 @@ fn main() {
             *prev
         } else {
             dense.push(chunk);
-            let new = match u8::try_from(chunkmap.len()) {
-                Ok(byte) => byte,
-                Err(_) => panic!("exceeded 256 unique chunks"),
+            let Ok(new) = u8::try_from(chunkmap.len()) else {
+                panic!("exceeded 256 unique chunks");
             };
             chunkmap.insert(chunk, new);
             new