Where 'small' means they don't refer to other metavariables, except
right at the top level, and they don't go beyond a certain small depth,
arrived at by experimenting.
We already did a bit of this, but only for depth 0. The effect of this
is that we don't need to save out lots of metavariables, so ttc loading
is faster. This takes about 8s off the Idris build time!
All of these internal names are making the output fragile. This
cleanup should allow us to only have to update the golden file
when there is a genuinely interesting change.
