Seek for stable

[PunitLodha]

Currently, on stable, seek doesn't work correctly
For seek implementation, https://github.com/meh/rust-ffmpeg/blob/master/src/format/io.rs#L93-L105, stream_len() is being used which is nightly only and 0 is returned in stable.
According to, rust-lang/rust#59359 and rust-lang/rust#58422, stream_len() is a convenience feature added for improved readability, which can be implemented manually by 3 seeks.

On stable, I think it would be better to manually implement using 3 seeks rather than, returning 0 which does not allow for a seek