asserts within library code should be opt-in not opt-out

[JAppleyard]

assert within device code is relatively expensive. While users can opt-in to disabling asserts by defining NDEBUG optimzied builds should not have to define this to get performant library code.

This would mirror libc++ which requires an opt-in: https://libcxx.llvm.org/docs/DesignDocs/DebugMode.html#id2