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.
Since they might be... This is especially likely for module hashes, and
if we don't get it right, the Racket runtime might fail to write the
buffer. This makes the code buildable with the Racket back end.
