Rollup merge of #150096 - Amanieu:revert-borrowedbuf, r=ChrisDenton

Revert #148937

rust-lang/rust#148937:  Remove initialized-bytes tracking from `BorrowedBuf` and `BorrowedCursor`

This caused several performance regressions because of existing code which uses `Read::read` and therefore requires full buffer initialization. This is particularly a problem when the same buffer is re-used for multiple read calls since this means it needs to be fully re-initialized each time.

There is still some benefit to landing the API changes, but we will have to add private APIs so that the existing infrastructure can track and avoid redundant initialization.

Author: matthiaskrgr