Minor improvements to `bounded_vec` and `defensive`. (#10873)

* Fix a few things in bounded_vec

* add test for try_extend

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

Co-authored-by: Bastian Köcher <[email protected]>

* some review comments

* use swap

* remove clone

* use pop instead of truncate

* remove warn

* review comments

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

Co-authored-by: Bastian Köcher <[email protected]>

* fix rustdoc

* fix links

* undo link

Co-authored-by: Bastian Köcher <[email protected]>