tomb: update API

- Rename Fatal and Fatalf to Kill and Killf.

- The Stop error value is gone. Use nil now.

- Err will now return ErrStillRunning if the goroutine isn't dead
  yet, which means a nil return reflects an actual nil Kill call.

- Make zero-value of Tomb good to use, and as a consequence, make
  Dead and Dying into methods rather than fields.

R=niemeyer
CC=
https://codereview.appspot.com/5755055
tree: d7435de38bf38a79136c4dc9ea569fc386e415ee
  1. LICENSE
  2. Makefile
  3. tomb.go
  4. tomb_test.go