std::io::prelude should stop renaming Seek

[lilyball]

std::io::prelude currently re-exports std::io::Seek as NewSeek. This should be changed before 1.0, as the renaming only makes sense in light of the old_io migration, and old_io is now deprecated. std::io::prelude is already declared stable, but I think that was a mistake and we should just go ahead and modify it.

This is related to #23377 (std::prelude::v1 still re-exports std::old_io::Seek).

/cc @alexcrichton

[pnkfelix]

1.0 beta, P-backcompat-libs. ( @alexcrichton says there's a PR in the queue.)