minor performance tuning.

directly write to io.Writer (i.e. v.Eval(w, ev)),
instead of w.Write(ev.Value(v))

before:
ukai@ubune.tok ~/src/kati% ./run_integration_test.rb android
Running make for android... 5.76 secs
Running kati for android... 13.38 secs
android: OK

PASS!

after:
% ./run_integration_test.rb android
Running make for android... 5.37 secs
Running kati for android... 12.65 secs
android: OK

PASS!
1 file changed