v.io/jiri/runtil: return Sequence and not *Sequence...

By returning a Sequence rather than a pointer to a Sequence
lines of the form:

if err := s.WriteFile(...); err != nil { ...

no longer compile. This prevents a common class of usability errors
and forces the use of err := s.WriteFile(...).Done() or
or s.Write(...).Run(...) etc.

MultiPart: 1/2

Change-Id: I7784ea4ff302d16f36a958f7ffe4140f366604a6
6 files changed