sum-types: Fix lint checking for case expressions

[RyanGlScott]

On the sum-types branch (#1602), if you write this:

// Foo.cry
enum Foo = MkFoo

f : Foo -> ()
f l =
  case l of
    MkFoo -> ()

// Foo.icry
:set coreLint=true
:l Foo.cry

Then running cryptol on Foo.icry will crash:

$ cabal run exe:cryptol -- -b Foo.icry 
Loading module Cryptol
Loading module Cryptol
{n, a, b, c} n == min n n
{a, n} (fin n) => inf == inf * (1 * (1 * 1))
{a, n} (fin n) => n / 2 == n - n /^ 2
{n, a, b} n == min n n
{n} (n >= 1, fin n) => (fin n, n >= 1)
Loading module Main
cryptol: src/Cryptol/TypeCheck/Sanity.hs:(212,3)-(338,56): Non-exhaustive patterns in case

We need to implement an ECase case in Cryptol.Typecheck.Sanity.exprSchema.