Rollup merge of rust-lang#147101 - yotamofek:pr/iter-eq-and-eq-by, r=jdonszelmann

Use `Iterator::eq` and (dogfood) `eq_by` in compiler and library

Now that rust-lang#137122 has landed, we can replace stuff that looks like:
```rust
let a: &[T];
let b: &[T];
let eq = a.len() == b.len() && a.iter().zip(b).all(|(a,b)| a == b)
```
with the much simpler `a.iter().eq(b)`, without losing the perf benefit of the different-length-fast-path.
Also dogfooded `Iterator::eq_by` (cc rust-lang#64295 ) while I'm at it.

First commit (4d1b6fa) should be very straightforward to review, second one (049a460) is slightly more creative, but IMHO a nice cleanup.

Author: Zalathar

alloc/src/collections/btree/map.rs

@@ -2413,7 +2413,7 @@ impl<K, V> Default for BTreeMap<K, V> {
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<K: PartialEq, V: PartialEq, A: Allocator + Clone> PartialEq for BTreeMap<K, V, A> {
     fn eq(&self, other: &BTreeMap<K, V, A>) -> bool {
-        self.len() == other.len() && self.iter().zip(other).all(|(a, b)| a == b)
+        self.iter().eq(other)
     }
 }