Make stream_len more atomic

[est31]

This makes stream_len atomic for File in an attempt to resolve the concerns brought up during the last stabilization attempt from which stream_len was ultimately removed.

This gives us atomic stream_len for all of the Seek implementations in std (or makes us use atomic stream_len if the underlying construct has such an atomic stream_len):

• Empty → already returns Ok(0) since f1cd179
• Cursor<impl Read> → already uses atomic operation since c518f2d
• File/&File → now uses atomic operation with this PR
• Box<impl Seek> → now uses inner stream_len
• &mut impl Seek → now uses inner stream_len
• BufReader<impl Seek> → now uses inner stream_len
• BufWriter<impl Seek> → now uses inner stream_len

The only non-atomic implementation remaining is the one on the trait itself, which is unfortunate, but I'm not sure if the alternatives to that are better or not.

cc @sfackler @LukasKalbertodt

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[the8472]

Tracking issue: #59359

This makes stream_len atomic for File in an attempt to resolve the concerns brought up during the last stabilization attempt from which stream_len was ultimately removed.

A comment further downthread suggested moving this method to a new trait so that it's only implemented on types that can return the length atomically.

[est31]

@the8472 mhh yeah good point. Closing.