Rollup merge of #57106 - matthiaskrgr:trim_must_use, r=sfackler
Rollup merge of #57106 - matthiaskrgr:trim_must_use, r=sfackler

Mark str::trim.* functions as #[must_use].

The functions return a reference to a new object and do not modify in-place
as the following code shows:
````
let s = String::from("   hello   ");
s.trim();
assert_eq!(s, "   hello   ");
````

The new reference should be bound to a variable as now indicated by #[must_use].

aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust