Rollup merge of #142331 - deven:trim_prefix_suffix, r=Amanieu

Add `trim_prefix` and `trim_suffix` methods for both `slice` and `str` types.

Implements `trim_prefix` and `trim_suffix` methods for both `slice` and `str` types, which remove at most one occurrence of a prefix/suffix while always returning a string/slice (rather than Option), enabling easy method chaining.

## Tracking issue
rust-lang/rust#142312

## API
```rust
impl str {
    pub fn trim_prefix<P: Pattern>(&self, prefix: P) -> &str;
    pub fn trim_suffix<P: Pattern>(&self, suffix: P) -> &str
    where
        for<'a> P::Searcher<'a>: ReverseSearcher<'a>;
}

impl<T> [T] {
    pub fn trim_prefix<P: SlicePattern<Item = T> + ?Sized>(&self, prefix: &P) -> &[T]
    where
        T: PartialEq;
    pub fn trim_suffix<P: SlicePattern<Item = T> + ?Sized>(&self, suffix: &P) -> &[T]
    where
        T: PartialEq;
}
```

## Examples
```rust
// Method chaining
assert_eq!(" <https://example.com/> ".trim().trim_prefix('<').trim_suffix('>').trim(), "https://example.com/");

// Slices
let v = &[10, 40, 30];
assert_eq!(v.trim_prefix(&[10]), &[40, 30][..]);
```

## ACP
Originally proposed in rust-lang/libs-team#597

Author: tgross35