Overlap between `IO#seek` and `#skip`

[straight-shoota]

IO has two methods #seek and #skip which share some common behaviour.
Every IO implements #skip, but only some do #seek: FileDescriptor and IO::Memory.

seek directly mutates the file position, whereas skip actually consumes data and discards it. So it only allows moving forward, while seek has more modes.
But with mode Seek::Current and a positive offset, both methods perform the same operation: move the current position forward by x:
skip(x) is equivalent to seek(x, :current) as long as x >= 0.

This could allow two improvements:

• Use skip as fallback implementation for seek(x, :current) (with x >= 0) for every IO.
• Optimize skip by delegating to seek when available. This should have a significant effect for large offsets where we can avoid many successive reads just to progress the position. It might not be an improvement for small offsets, though (especially with IO::Buffered), so the implementation would need to consider some kind of threshold.

[HertzDevil]

IO::FileDescriptor#seek always flushes its read buffer, whereas IO::Buffered#skip may advance the buffer without flushing if x is small enough, so the former is not necessarily an optimization over the latter.