Type checker returning spurious undefined function errors

[tsandall]

The type checker requires that the caller sort the rules topologically however for independent rules (p and q below) there is no ordering requirement (and the ordering for independent rules is currently non-deterministic.) This results in spurious undefined function errors. For example:

package x

p { 
  concat("", 1)  # type error
}

q {
  f(1)
}

f(x) = x

The spurious "undefined function" errors are emitted because the type checker does not update the type environment once it finds a single error AND the order that independent rules are checked in is non-deterministic. For example:

case 1:

f  # no error, f added to type env
q  # no error, q added to type env
p  # concat type error, p not added to type env

case 2:

p  # concat type error, p not added to type env
f  # no error, but since checker contains error for p, f not added to type env
q  # undefined function error since f not in type env

This is related to #2091 but not quite the same. It's possible that the solution for this issue could resolve #2091. For example, the type checker could be updated to add rules/functions to the type env EVEN IF they contain errors. Since the types of terms in the head of the rule can't be inferred reliably in this case, they could be added as ANY types. This would mean dependents of erroneous rules/functions would not encounter "undefined function" or "undefined ref" or other type errors. This would be nice because independent rules could still be type checked and we can report the maximum number of errors in one pass.