Optimize ninja_writer

Prebuild a string of all spaces to avoid calling strings.Repeat for
every indented line.  Replace appendWithWrap function with a class
that handles counting bytes on a line as they are written instead of
buffering lines, and avoids extra string concatenations by providing
a function that implicitly adds a space.  Saves 10% wall time on one
workload.

Change-Id: I571d70366ee5db21c40c1c98d0fe92d640efefab