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.
For the same behaviour as Idris 1, the primitive cast should return 0 if
the integer is out of bounds. (We should probably drop the Cast
implementation though, since ideally they won't be lossy in general, but
that's an issue for another time...)
All the tests pass in racket now, for me.
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.
