Rollup merge of #49559 - djc:resize-with, r=TimNN

Introduce Vec::resize_with method (see #41758)

In #41758, the libs team decided they preferred `Vec::resize_with` over `Vec::resize_default()`. Here is an implementation to get this moving forward.

I don't know what the removal process for `Vec::resize_default()` should be, so I've left it in place for now. Would be happy to follow up with its removal.

diff --cc src/liballoc/vec.rs
Simple merge