Rollup merge of #42227 - ollie27:into_to_from, r=aturon

Convert Intos to Froms.

This is a resubmission of #42129 without `impl<T> From<Vec<T>> for Box<[T]>`.

src/liballoc/string.rs

@@ -2008,10 +2008,10 @@ impl From<Box<str>> for String {
     }
 }
 
-#[stable(feature = "box_from_str", since = "1.18.0")]
-impl Into<Box<str>> for String {
-    fn into(self) -> Box<str> {
-        self.into_boxed_str()
+#[stable(feature = "box_from_str", since = "1.20.0")]
+impl From<String> for Box<str> {
+    fn from(s: String) -> Box<str> {
+        s.into_boxed_str()
     }
 }