Auto merge of #21806 - edwardw:new-range-impl, r=alexcrichton

The new `::ops::Range` has separated implementations for each of the
numeric types, while the old `::iter::Range` has one for type `Int`.
However, we do not take output bindings into account when selecting
traits. So it confuses `typeck` and makes the new range does not work as
good as the old one when it comes to type inference.

This patch implements `Iterator` for the new range for one type `Int`.
This limitation could be lifted, however, if we ever reconsider the
output types' role in type inference.

Closes #21595
Closes #21649
Closes #21672

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

@@ -17,10 +17,7 @@ pub fn main() {
 
     // Float => does not implement iterator.
     for i in 0f32..42f32 {}
-    //~^ ERROR `core::iter::Iterator` is not implemented for the type `core::ops::Range<f32>`
-    //~^^ ERROR
-    //~^^^ ERROR
-    // FIXME(#21528) not fulfilled obligation error should be reported once, not thrice
+    //~^ ERROR the trait `core::num::Int` is not implemented for the type `f32`
 
     // Unsized type.
     let arr: &[_] = &[1u32, 2, 3];