Rollup merge of #81154 - dylni:improve-design-of-assert-len, r=KodrAus

Rollup merge of #81154 - dylni:improve-design-of-assert-len, r=KodrAus

Improve design of `assert_len`

It was discussed in the [tracking issue](https://github.com/rust-lang/rust/issues/76393#issuecomment-761765448) that `assert_len`'s name and usage are confusing. This PR improves them based on a suggestion by ``@scottmcm`` in that issue.

I also improved the documentation to make it clearer when you might want to use this method.

Old example:

```rust
let range = range.assert_len(slice.len());
```

New example:

```rust
let range = range.ensure_subset_of(..slice.len());
```

Fixes #81157