* Make _sanityCheck internal
* Make _debugPrecondition internal
* Make Optional._unsafelyUnwrappedUnchecked internal.
* Make _precondition internal
* Switch Foundation _sanityChecks to assertions
* Update file check tests
* Remove one more _debugPrecondition
* Update Optimization-with-check tests
* Remove case destructuring to _
* Remove some Iterator.Element
* Which idiot wrote this? Oh.
* Switch NibbleSort to just use default impls... shouldn't change perf
StdlibUnittest uses gyb to avoid duplicating many source-context
arguments. However, this means that any test that wishes to add new
expect helpers has to also be gybbed. Given that this structure hasn't
changed in years, and we should have a real language support
eventually, de-gyb it.
