io::BorrowedCursor::advance
This also keeps the old `advance` method under `advance_unchecked` name. This makes pattern like `std::io::default_read_buf` safe to write.
BorrowedBuf
BorrowedCursor
std:io
core::io
Assigned new feature name `core_io_borrowed_buf` to distinguish from the `Read::read_buf` functionality in `std::io`.