use kit reader, not chdir, to get kit contents

**tl;dr**:  I'd like to switch our work mkits from TT2 to MicroMason, but that isn't going to be possible until MicroMason is made to turn filenames into kit entries, rather than files inside the kit.  We use the SWAK kitreader, which does more than look inside the kit dir.

Commit message:

Changing to the kit dir before rendering is a bit of a dirty hack,
but it works... as long as the kit's kitreader is Dir.  If you're
using something more complicated, like KitReader::SWAK, it doesn't
actually help.

Instead of doing that, this commit adds a mixin to the micromason
object, converting its reads of files-by-name into reads of kit
contents.

Admittedly, the way that the renderer is shoved into the mason
object is also a dirty hack, but presumably it's one that can be
corrected by someone more familiar with how micromason mixins are
meant to work.