Stream files

[NathanBaulch]

I noticed a few places where files were being generated with StringBuilder in memory before writing to the FS.
Better to use StreamWriter and write directly to the FS rather than consuming unnecessary memory.
It's also a bit more readable and less error prone using WriteLine instead of \n IMO.
I've left the string interpolation in place because the code reads a little better. In theory those could be broken into multiple calls to Write to avoid even more string building behind the scenes.