rust version: backtracking simplified and other improvements

* backtracking: main loop simplified
* instead of the `fixed` Vec, we now use skip fields to calculate
  how many fields to skip in `next()` and `prev()`.
  This is faster because we get rid of the `if` statement
  thus no branching occurs.

Cargo.toml

@@ -9,5 +9,6 @@ edition = "2021"
 [profile.release]
 lto = true
 debug = false
-codegen-units = 1
+strip = true
 panic = "abort"
+codegen-units = 1