Improve BoundedVec API (extracted from #10195) (#10656)

* Gav wrote this code in pull #10195. Extracting to simplify that PR.

* fix potential panics

* prevent panics in slide

* update doc

* fmt

* Update frame/support/src/storage/bounded_vec.rs

Co-authored-by: Keith Yeung <kungfukeith11@gmail.com>

Co-authored-by: Gav Wood <gavin@parity.io>
Co-authored-by: Keith Yeung <kungfukeith11@gmail.com>