Fix range performance regression

A recent change to the implementation of range iterators meant that,
even when stepping by 1, the iterators *always* involved checked
arithmetic.

This commit reverts to the earlier behavior (while retaining the
refactoring into traits).

Fixes #24095
cc #24014

src/test/compile-fail/range-1.rs

@@ -19,6 +19,7 @@ pub fn main() {
     for i in false..true {}
     //~^ ERROR the trait
     //~^^ ERROR the trait
+    //~^^^ ERROR the trait
 
     // Unsized type.
     let arr: &[_] = &[1, 2, 3];