Rollup merge of #73197 - c410-f3r:ranges, r=dtolnay

Impl Default for ranges

Couldn't find an issue about it.

`Range` and friends probably can implement `Default` if `Idx: Default`. For example, the following would be possible:

```rust
#[derive(Default)]
struct Foo(core::ops::RangeToInclusive<u64>);

let _ = [1, 2, 3].get(core::ops::Range::default());

core::ops::RangeFrom::<u8>::default().take(20).for_each(|x| { dbg!(x); });

fn stuff<T: Default>() { let instance = T::default(); ... more stuff }
stuff::<core::ops::RangeTo<f32>>();
```

Maybe there are some concerns about safety or misunderstandings?

Author: Manishearth

Diff for: src/libcore/ops/range.rs

@@ -39,7 +39,7 @@ use crate::hash::Hash;
 /// [`Iterator`]: ../iter/trait.IntoIterator.html
 /// [slicing index]: ../slice/trait.SliceIndex.html
 #[doc(alias = "..")]
-#[derive(Copy, Clone, PartialEq, Eq, Hash)]
+#[derive(Copy, Clone, Default, PartialEq, Eq, Hash)]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct RangeFull;
 
@@ -71,7 +71,7 @@ impl fmt::Debug for RangeFull {
 /// assert_eq!(arr[1..=3], [  1,2,3  ]);
 /// ```
 #[doc(alias = "..")]
-#[derive(Clone, PartialEq, Eq, Hash)] // not Copy -- see #27186
+#[derive(Clone, Default, PartialEq, Eq, Hash)] // not Copy -- see #27186
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct Range<Idx> {
     /// The lower bound of the range (inclusive).