The key idea is that we define:

    type IO a = HasCallStack => Prelude.IO a

and voila, call stacks are maintained across all IO!  You can
look at the stacks using -v"debug +callstack".

There are a number of IO functions for which the call stack is
never used.  They are explicitly annotated using NoCallStackIO.
Maybe some day they will use call stacks and we can change their
types.  Similarly, there are a number of functions which do
have type IO, but then suppress the redundant constraint error
using "_ = callStack". Maybe some day we will attach call
stacks to the exceptions we throw.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>