The typechecker previously passed around a boolean return flag to
indicate whether it saw something with type _|_ (that is, something
it knows at compile-time will definitely diverge) and also had some
manual checks for the `ty_err` pseudo-type that represents a previous
type error. This was because the typing rules implemented by the
typechecker didn't properly propagate _|_ and ty_err. I fixed it.
This also required changing expected error messages in a few tests,
as now we're printing out fewer derived errors -- in fact, at this
point we should print out no derived errors, so report any that
you see (ones that include "[type error]") as bugs.
Autoderef on binops is basically unused, kind of silly, and
complicates typechecking. There were only three instances of it in the
compiler and the test drivers, two of which were of the form "*foo =
foo + 1", which should be written as "*foo += 1" anyways.
