sum-types: Surprising interaction between default cases and cases that follow it

[RyanGlScott]

On the sum-types branch (#1602), Cryptol will compile the following code without any warnings:

enum Foo = A | B

f : Foo -> Bit
f l =
  case l of
    _ -> True
    B -> False

Then f A will evaluate to True and f B will evaluate to False. This is not desirable, since we'd like _ to be a catch-all case that covers all patterns not listed above it. We should change the behavior so that:

1. Both f A and f B evaluate to True.
2. Cryptol emits a warning that the _ case overlaps with the B case.

[RyanGlScott]

Actually, the sum-types branch makes overlapping patterns an error rather than a warning. For consistency, I'll change this proposal to just be:

1. f is rejected with an error saying that the B case overlaps with the _ case.

There's no point in speculating what f B evaluates to in this case, since f won't compile in the first place.

[RyanGlScott]

Fixed in ca85fac.