Rollup merge of #125087 - tbu-:pr_file_stream_len, r=ChrisDenton

Optimize `Seek::stream_len` impl for `File`

It uses the file metadata on Unix with a fallback for files incorrectly reported as zero-sized. It uses `GetFileSizeEx` on Windows.

This reduces the number of syscalls needed for determining the file size of an open file from 3 to 1.

Author: matthiaskrgr