Still a couple of things to resolve in coverage and totality checking
before we can switch on %default, so don't expect quite the right
behaviour just yet. More progress though!
Also working on this has caught a few totality errors in the Idris 2
code base that Idris 1 missed... so these are fixed on the way.
Racket appears to have a different notion of current directory than the
system does, so we need to tell it which directory we think we're in
when reading and writing bytevectors using the scheme file functions.
