Finalize the Seek API

This adopts the rules posted in #10432:

1. If a seek position is negative, then an error is generated
2. Seeks beyond the end-of-file are allowed. Future writes will fill the gap
   with data and future reads will return errors.
3. Seeks within the bounds of a file are fine.

Closes #10432
auto merge of #12204 : alexcrichton/rust/seek, r=pcwalton

This adopts the rules posted in #10432:

1. If a seek position is negative, then an error is generated
2. Seeks beyond the end-of-file are allowed. Future writes will fill the gap
   with data and future reads will return errors.
3. Seeks within the bounds of a file are fine.

Closes #10432