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/config.toml

@@ -1,3 +1,2 @@
 [build]
-target = "x86_64-unknown-linux-musl"
-rustflags = "-C target-cpu=native -C strip=symbols"
+rustflags = ["-C", "target-cpu=native", "-C", "target-feature=+crt-static"]