Delay evaluation of it blocks until test execution

[zyla]

Motivation:

Sometimes even pure code can have crashes (example: messing up the FFI when binding to a JavaScript library). I'd argue that test suites should deal with that, as they are a development tool.

Consider this test case:

spec :: Spec Unit
spec = do
  it "failure" do 
    unsafeCrashWith "failure"

Any program containing this definition will crash while loading the module, before it even calls main, because unsafeCrashWith is not under any lambda, so it's evaluated immediately. The same example would work just fine (i.e. the crash would be caught by test runner) in Haskell and hspec, thanks to lazy evaluation.

We can emulate the desired behavior using a hack that effectively wraps every it block in a lambda in the generated code.

This would be a breaking change. "Normal" test code should compile just fine, but code that uses it in a generic way may need some eta-expansion to deal with the constrained type.